Michelangelo, Weirdness to 11

Mutagen

When Michelangelo enters, create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")
If one or more +1/+1 counters would be put on a creature you control, that many plus one +1/+1 counters are put on it instead.

Leonardo, Sewer Samurai

Sneak {2}{W}{W}
Double strike
During your turn, you may cast creature spells with power or toughness 1 or less from your graveyard. If you cast a spell this way, that creature enters with a finality counter on it. (If a creature with a finality counter on it would die, exile it instead.)

Donatello, Mutant Mechanic

{T}: Put three +1/+1 counters on target artifact you control. If it isn't a creature, it becomes a 0/0 Robot creature in addition to its other types. Activate only as a sorcery.
Whenever an artifact you control is put into a graveyard from the battlefield, if it had counters on it, put those counters on up to one target artifact or creature you control.

Super Shredder

Menace
Whenever another permanent leaves the battlefield, put a +1/+1 counter on Super Shredder.

Raphael, Ninja Destroyer

Raphael must be blocked if able.
Enrage — Whenever Raphael is dealt damage, add that much {R}. Until end of turn, you don't lose this mana as steps and phases end.

Michelangelo, Improviser

Sneak {2}{G}{G} (You may cast this spell for {2}{G}{G} if you also return an unblocked attacker you control to hand during the declare blockers step. He enters tapped and attacking.)
Whenever Michelangelo deals combat damage to a player, you may put a creature card and/or a land card from your hand onto the battlefield.

Prehistoric Pet

This creature can't be blocked by creatures with greater power.
{1}{W}, {T}: Return another target creature you control to its owner's hand. Activate only during your turn.

Ninja

Dark Leo & Shredder

Sneak {W}{B}
Attacking Ninjas you control have deathtouch.
Whenever Dark Leo & Shredder deal combat damage to a player, create a 1/1 black Ninja creature token. Then if you control five or more Ninjas, that player loses half their life, rounded up.

Krang, Utrom Warlord

Flying, trample, indestructible, haste
Other artifact creatures you control have flying, trample, indestructible, and haste.

Technodrome

Reach, trample
This creature can't attack or block unless its power is 6 or greater.
{T}, Sacrifice another artifact: Draw a card. Put a +1/+1 counter on this creature.

The Last Ronin's Technique

Ninja Turtle Spirit

Sneak {1}{W} (You may cast this spell for {1}{W} if you also return an unblocked attacker you control to hand during the declare blockers step.)
Create three 1/1 white Ninja Turtle Spirit creature tokens. If this spell's sneak cost was paid, they enter tapped and attacking.

Leonardo's Technique

Sneak {1}{W} (You may cast this spell for {1}{W} if you also return an unblocked attacker you control to hand during the declare blockers step.)
Return one or two target creature cards each with mana value 3 or less from your graveyard to the battlefield.

Mutant

Sally Pride, Lioness Leader

When Sally Pride enters, create X 2/2 red Mutant creature tokens, where X is the number of nontoken creatures you control.
Whenever Sally Pride attacks, put a +1/+1 counter on each creature you control.

Triceraton Commander

Dinosaur Soldier

Flying
Whenever this creature attacks, Dinosaurs you control other than this creature get +1/+1 and gain flying until end of turn.
When this creature enters, create X 2/2 white Dinosaur Soldier creature tokens.

April O'Neil, Hacktivist

At the beginning of your end step, draw a card for each card type among spells you've cast this turn.

Donatello's Technique

Sneak {U} (You may cast this spell for {U} if you also return an unblocked attacker you control to hand during the declare blockers step.)
Draw two cards.

Teenage Mutant Ninja Turtles