I got stuck doing As You Like It this year, not fun.
Anyway, Celia:
- She has extreme devotion to Rosalind to follow her into exile, so they belong to each other.
- She originally doesn't belong in the Forest of Arden so she assumes a disguise of Aliena (Latin for stranger.. you can play on that a bit.) in an attempt to establish her identity and a sense of belonging
- At the end of the play after the marriages by Hymen, she belongs to Oliver through marriage.
Oliver:
- Originally Oliver isolates himself from his brother Orlando, however after Orlando saves his life he has a change of heart and belongs with his brother through loyalty.
- Belongs with Celia by marriage (obviously).
I'm not too good with Oliver as you can see, but there's a bunch of other things to explore with Celia and ways you can elaborate with both characters, etc.
Good luck.
Edit: Also try posting in the English AOS forum under 'HSC' a little further down the page. This topic will get a lot more notice down there