They always have a different color tint and increased size (although some bosses might keep their size or even shrink).
Champion monsters have double health, possess larger shots, and deal a full heart of damage instead of half. They always drop a Pick Up when killed.
Champion bosses come with more drastic differences: Aside from increased damage, some might even possess new abilities (the champion Larry Jr. gaining the ability to spit shots, for example) or are otherwise more threatening (Monstro might appear with a duplicate, and Peep can start with both eyes removed). Faster movement (like Gemini) is also common, or a change in spawning behavior (Gurdy might just spawn instead of shooting, and Chub might also spawn Spities or Boils instead of Chargers).
Champion forms of bosses will always drop a heart upon defeat when found outside a boss room or an extra heart when found inside a boss room. If the champion boss is blue, the heart(s) dropped will instead be a Soul Heart.
Rebirth Champion monsters, along with larger health and damage, possesses additional abilities, which depends on Champion color.
- Green - leaves green creep while moving. Drops a pill
- Red - has increased health. Drops a red heart
- Dark Red - when killed it leaves behind goo. If the goo is not killed the champion revives, like a globin. Drops a double heart. Works with certain suicide moves, like Mulligan self-exploding, but doesn't stack with Globin native ability.
- Pink - randomly shoots a projectile in the direction it is facing. Drops nothing
- Yellow - moves considerably faster. Drops Lil Battery
- Grey - has decreased health. Drops key.
- Dark Grey - explodes after death. Drops a bomb.
- Light Blue - shoots eight projectiles upon death. Drops 1/2 heart
- Dark Blue - gives three blue flies after death
- Gold - steals money from you when it hurts you. Drops a coin.
- Black - randomly turns invisible. Drops a red chest.
- Purple - pulls in you and your shots. Drops a random trinket.
- Transparent - can move over any object. Drops a golden chest.
- White: is invincible until everything in the room is killed. Drops an eternal heart
Some colors can be distorted, since champion color mixes with common enemy color.
- When leaving a room and reentering before defeating all the monsters or bosses, the chance of a monster or boss being a champion form is randomized again.
- It is possible to meet two champions with the exact same color in the same room.
- Certain monsters, like the Evil Twin, are considered Champions by the game in The Womb, and will drop a Pick Up upon death.
- A lot of new champion bosses were added in the v1.4 update, including champion forms of bosses that previously did not have one. This includes all Wrath of the Lamb bosses as well as bosses from the basic game, like The Harbingers or even Mom.
- Champion versions of enemies could be a reference to Diablo II, which has champion enemies that have different colors, and do more damage.