Well he does fit better with Sheryl on personality anyways. The problem with AltoxRanka is that they're both too shy to go anywhere and Alto would basically not acknowledge it while Ranka would be blushing too much to do anything else. Sheryl is a better fit in that she can sort of force him to be honest, i.e. by having him come and save her etc. and forcing him to do things since she's outgoing...
For the people who've seen Kimikiss basically:
AltoxRanka = KouichixYuumi
AltoxSheryl = KouichixMao
Personally I like both Ranka and Sheryl, but I think Ranka needs Alto more as a guy who can support her, whereas Sheryl needs him romantically.