Shirou has developed many relationships throughout the series within the Occult Research Club, Stuident Council, and the Three Factions. He has also attracted the attention of many of the female characters.
Occult Research Club
Issei Hyoudou
Issei is Shirou's best friend and mutually proclaimed "blood brother" as well as Shirou's rival. Shirou and Issei have known one another since they were little, Issei having been one of Shirou's first friends, being the one who drew Shirou out of his depression following his mother's death and again when Irina, Shirou's first love, moved away. Issei made a habit of becoming Shirou's personal protector from their childhood through middle school. Shirou deeply cares for Issei and desires to protect Issei just as Issei protected him.
Shirou also desires to return Issei to a semblance of "normal", wishing to calm Issei's perverted instincts. While mostly ineffective, Shirou's repeated lessons have slowly begun to sink into Issei's mind, helping Issei display a more romantic attitude towards building a harem. Due to Shirou's superior instincts, he is quite capable at sensing the affection Rias has for Issei and secretly plots to push the two together to ensure their happiness.
However, while Shirou deeply cares for Issei, he secretly possesses a deep-seeded sense of inferiority compared to Issei, further agitated upon realizing Issei had won more important fights between the two. Shirou does not hate Issei, but rather, Shirou deeply envies Issei and what he perceives to be Issei's superior strength, subconsciously believing himself to be pathetic and unnecessary.
At the same time, Issei is Shirou's most important emotional support. Neither truly realize it, but Issei is a major pillar holding up Shirou's emotional state. This is to the point that Shirou will risk death in the most reckless choice in order to save and protect Issei, literally sacrificing himself first to protect Issei from Raynare's attempted assassination. Shirou places a great amount of care on Issei's happiness and safety even to the point of costing his own. If Issei were to be killed, Shirou would most likely fall into an inescapable depression, succumbing to his negative emotions and simply "giving up on life", dying as a result.
Issei also deeply cares about Shirou, trying to do what would be in his best interest. He is also the best at reading Shirou's thoughts, knowing when something is troubling him or knowing what it is that Shirou would want or feel. This is demonstrated during the battle with Kokabiel when, after he and everyone else thought Shirou had been killed by Kokabiel, he refused to succumb to his rage, knowing Shirou would not forgive him if he did not put protecting their friends and loved ones as his first priority.
Rias Gremory
Shirou and Rias have been shown to have a good relationship, the two speaking on a casual first-name basis. Shirou is the first one to address Rias's romantic feelings for Issei and encourages them. Rias shows some formal respect for Shirou as Serafall Leviathan's Queen and fears Serafall's retribution should serious harm come to Shirou, but their relationship is generally casual. However, this does not stop Rias from threatening to castrate Shirou if he ever abuses or hurts Asia's feelings.
They share mutual feelings of friendship, Rias having shed tears when she thought Shirou had been killed by Kokabiel.
Asia Argento
Asia's affection for Shirou started out as a misunderstanding, Shirou's poor choice of words leading Asia on to believe that he had been proposing to her. Shirou initially felt uncomfortable with Asia's affections due to the misunderstanding, but could not correct her due to Rias's repeated threats of castration if he were to hurt Asia's feelings. Shirou later came to reciprocate Asia's love for him after she proved herself a great emotional support in Volume 3, saving Shirou from suffering a complete mental breakdown, later becoming Shirou's lover along with Ravel.
Akeno Himejima
Shirou and Akeno are on uncertain yet close terms. Shirou is uncertain of Akeno's true feelings for him but counts her as a dear friend despite often being embarrassed or exhausted by Akeno's teasing and "S-side". It is strongly suggested that Akeno harbors deeper affections for Shirou.
Yumi Kiba
Shirou and Yumi are classmates and close friends, later becoming lovers as a result of Yumi's long-standing love for Shirou.
Koneko Toujou
Koneko and Shirou are on good terms despite Koneko thinking of Shirou as "lower than Ise" due to his rather poor victory record along with his clear perversion. However, it appears that, lately, Koneko has been acquiring some jealousy and often takes out her frustration on Shirou, Koneko herself not understanding her reasoning.
Raynare
An "honorary" Occult Research Club member serving as Shirou's maid. Despite having been his would-be murderer, Raynare is counted amongst Shirou's friends due to her change of character under the influence of the ORC. Raynare has begun developing deeper feelings for Shirou, initially being lust which later develops into love upon hearing Shirou's dream to create a peaceful world for all factions.
Kalawarner
An '"honorary Occult Research Club Member serving as Shirou's maid. Kalawarner was the quickest (second behind MIttelt) to accept servitude to Shirou, acting as something of a caring older sister to her fellow Fallen Angels. Kalawarner enjoys teasing and flirting with Shirou, but her true feelings are unclear. She and Shirou generally possess a good relationship.
Mittelt
An '"honorary Occult Research Club Member serving as Shirou's maid. Mittelt is deeply in love with Shirou, quickly being charmed by his kind personality despite the fact they were originally enemies. Shirou cares about Mittelt, being grateful towards her for her help in proving the ignorance of Raynare's group and preventing their executions at the hand of Rias.
Isabela
Isabela is Shirou's Rook, having been won in a wager Shirou had with Riser along with several of Riser's other servants. Isabela was one of the first few to warm up to Shirou during his short time as Ravel's servant, being one of the few he could truly call his friend amongst his former enemies. Isabela respects Shirou and volunteered to become his Rook so she could follow him and become stronger. Shirou accepted Isabela as his servant because he is aware of her growing affection for Issei, Shirou plotting to push the two together at the same time he is trying to push Issei and Rias together, covertly trying help Issei jump-start his harem.
Ravel Phenex
Ravel is Shirou's Bishop, having been won in a wager Shirou had with Riser along with several of Riser's other servants. Ravel had fallen in love with Shirou at first sight despite his attempts to charm her having been more of a teasing gesture to spite Riser. She could be said to have been obsessed with Shirou, having coerced Riser into accepting his (really Serafall's) wager even if losing would have cost him half of his Peerage. After Riser's victory against Rias, Ravel temporarily became Shirou's King, her feelings increasing as her obsession became even stronger. Her obsession came to a head when she attempted to force herself upon Shirou at the threat of having him killed, having overheard and misinterpreted words Shirou spoke before, believing Shirou had no care for her and thought of her as nothing more than a tool to help him return to Serafall's side.
They later reconciled, Ravel beginning to realize Shirou's true feelings for Serafall and that her actions were only hurting the boy she fell in love with. After a talk with Isabela, Ravel gave up her own desires to keep Shirou from experiencing further pain, exploiting a loophole in the wager with Riser to help Shirou return to Serafall. This single kind act earned Shirou's affection, the two's relationship becoming much more positive.
Ravel later became an emotional support of Shirou as well as his second lover along with Asia.
Xenovia
Shirou and Xenovia originally despised one another, but later developed a mutual respect for one another after experiencing several battles together. Xenovia later came to deeply respect and admire Shirou after learning of God's death, especially after learning of Shirou's dream to create a peaceful world for everyone. She refers to Shirou with the "-sama" honorific. She appears to have genuine romantic affection for Shirou, wanting to bear his child not just for his Satan bloodline, but for Shirou's indomitable will and powerful spirit.
Student Council
Sona Sitri
Shirou and Sona possess a friendly relationship, both sympathizing with each other over the fact they are easily exhausted by Serafall's energy and childish antics. Sona shows some care for Shirou since he was placed under her to be taught the ways of the Devils, Sona showing a fair degree of protectiveness over Shirou, knowing Serafall would cry if Shirou were hurt or killed. Sona has openly expressed some envy over Shirou's ability to remain hopeful in even the most despairing circumstances and at the same time admires Shirou. It is somewhat hinted that she may care more for Shirou than she lets on.
Tsubaki Shinra
Shirou and Tsubaki are good friends. Shirou enjoys teasing Tsubaki and deeply enjoys her flustered reactions to his flirting. It is hinted that Tsubaki may be developing feelings for Shirou
Tsubasa Yura
Tsubasa is one of Shirou's self-proclaimed harem girls, possessing deep affection for Shirou. Shirou also recently began developing feelings for Tsubasa, being someone he deeply cares for. It is implied that Tsubasa is amongst Shirou's more avid stalkers, knowing where Shirou lives despite Shirou never having led here there.
Ruruko Nimura
Shirou's first harem girl and "official girlfriend". The two love each other very much and, despite Ruruko's jealousy, She often provides support and advice to Shirou whenever he's troubled with life or women.
Momo Hanakai
Momo is deeply in love with Shirou and is Ruruko's main rival. However, despite seeming harsh and selfish, Momo actually does care for both Shirou and Ruruko, having stepped back so Ruruko could confess to Shirou, ultimately allowing their relationship. However, now that Shirou is desiring to become a Harem King and since Ruruko is already one of his women, Momo had decided that all gloves were off.
Fujimura Household
Makoto Fujimura
Makoto is Shirou's older sister. The two are awkward at expressing their sibling love and tend to express it through minor fist fights. Makoto became Shirou's legal guardian after Shirou's mother, Miyako Fujimura, died ten years ago. Makoto is, in fact, only related to Shirou through their father.
Jack
Jack is a girl with an unknown past as well as Shirou's Familiar. She is classified as an "Evil Spirit", but tends to display a more innocent disposition, especially around Shirou whom she refers to as her "Onii-chan". Jack appears to have some connection to Shirou, but Shirou possesses no memories of her prior to encountering her in the Familiar Forest.
Devils
Serafall Leviathan
Serafall is Shirou's King and first lover. They are deeply in love with one another.
Sirzechs Lucifer
Shirou and Sirzechs have only met twice, both times, Shirou had not spoken a real word with him, but Shirou respects him greatly. He later on develop a hatred for him because he decided to have Shiro executed because of his blood.
Grayfia Lucifuge
Shirou and Grayfia have only met a few times, but they appear to respect one another as fellow Queens of the Maous.