I don't like using characters that are given to you and are already tier 2-- usually their stats are worse than what you can get by levelling up some characters from the earlygame. That aside, I always supported Roy to Lilina or Sophia, since having a mage beside your lord is never a bad idea and isn't necessarily tough to keep track of. (Whereas, if you were to marry Cecilia, since she's a mounted unit she would move around faster than Roy does.) So it's like, not only having the caster+staff user next to your lord, but also not limitting a mounted unit to 5/6 spaces opposed to 7/8.
In FE 7 I also always married Eliwood to Ninian for the ending, so technically Roy is half ice dragon from his mother. :333 The more you know. (This is also canon because it's in the Fire Emblem Novels, so yeah, Roy is half ice dragon even though his blade is designed around fire.)