Most Important Supporting Characters

[CylonsKlingonsDaleksOhMy]

This new movie and the botched (non-existent) Yoshi/Saki/Splinter backstory got me thinking... Who do you think are the most important supporting characters to the four Turtles? Who else is a vital part of their story, no matter what series or medium? I've listed my personal take on this in order of importance, but I'm interested to see who else agrees or disagrees...

1. Splinter/Hamato Yoshi
Even though they're not always the same character, I'm listing them as one. Splinter is the adoptive father and sensei to the Turtles, having trained them in ninjitsu. His link to Hamato Yoshi (whether as his former human self or as the man's former pet rat) is what ties the Turtles to the Foot Clan.

2. Oroku Saki/The Shredder
Leader of the Foot Clan and rival to Splinter/Yoshi and determined to destroy the mutant rat and turtles because of their ties to Hamato Yoshi and the resulting blood feud. The archenemy, without whom the story of the Turtles is incomplete.

3. April O'Neil
(Usually) the first human to meet the Turtles and accept them as friends, she has always been a part of the family and acted as either a sister or mother figure.

4. Utroms
I'm listing the race instead of any particular individual as it is always the Utroms' presence on Earth that introduces the mutagen that creates the Turtles. (Except in the Fred Wolf cartoon, where Krang is responsible for the mutagen and is only loosely based on the Utroms.) Their ties to the mutagen usually isn't introduced first, however, which is why I consider April more important.

5. Baxter Stockman
Baxter has usually served as the introduction for most of the non-terrestrial technology in TMNT, most notably the Mousers and lately also directly responsible for the Turtles' mutation as well. He also has almost always served as an antagonist to April O'Neil at one level or another...