Link - legacy character using common items with multiple distinct costumes. Default costume Skyward Sword.
Link - the younger version, default costume "Toon Link" without the ridiculous prefix. Alternate costumes for "Young Link" and "Link to the Past Link"
Zelda - default costume Skyward Sword, with others available. Still transforms into Sheik, but Sheik's costume doesn't ever change.
Ganondorf - default costume Twilight Princess, but Wind Waker also available.
Tetra - standalone. None of this "Toon Zelda" transformation nonsense. (I mean seriously, she attacks Ganondorf bareknuckled as Tetra, then gets kidnapped the instant she becomes Zelda. Sure she fires arrows as Zelda, but she fired CANNONS as Tetra.)
I'm not convinced that the Zelda series needs any more characters, but if so, I'd prioritise them:
Vaati
Impa
Ghirahim
Linebeck