Which characters? There's four that have Hallowe'en outfits (Pixie, Gwen, Chloe and Jessica), and Jessica has two "hidden" outfits on top of that.
There's no "Easter" themed outfits as weekly release schedules don't really allow for seasonal outfits - I only included Hallowe'en stuff as an excuse the get the characters into sexy and unusual costumes :)
If it's the Hallowe'en ones you're missing then:
1. Buy and activate all three Hallowe'en decorations in your room (coffin bed, jack o' lantern on the beside table and the witch poster).
2. For Pixie: just sleep. Might take a few nights.
3. For Jessica and Gwen: make sure you own the Manor and have purchased the "Dungeon" upgrade, then go to the Dungeon on any evening.
Hope that helps! :)