She met Issei at the playground when they were in preschool. Apparently she was having a hard time fitting in because she's too shy. So Issei approached her and ask if she wants to be his friend and play with him. So those two became bestfriends and sworn to each other that they will best friends forever. During their middle school years, he developed a crush/feelings for her while Otome doesn't have a single clue about it because technically she does not know about love yet. When she's in US she realize that how much she misses Issei and for some reason he's always on her mind (hohohoho XD). During their reunion she felt joy and was glad that she gets to live in Japan with Issei again. Otome might have feelings for Issei but she notices about Rias and his relationship so with the other girls from the harem. Plus she knows Issei loves Rias but according to Issei he told Otome that he's in love with two girls. First one is Rias and he left Otome unanswered for the second one even though he's talking about her.
Friends. He was the one who tours her around the school. Many people thinks that those two are dating but they don't have feelings for each other (or maybe they will). They went on a date a couple of times.