I think the most important thing about Tomoe for me is that she has really good judgement. She likes to tease, but she knows when to stop and knows when she's crossed the line. With a lot of the other girls it feels like they are either Ryo's "superior" / senior (Alice, Keiko, Hana, Akane) or his dependant / pet (Ryoko, Shika, Charlotte, Fumiko, Mayu, maybe Yui). Tomoe feels like an equal. And she also is neither needy or clingy nor emotionally distant. I don't know, there's something about her that makes her feel like soulmate material.