Ettin

An ettin on the party's side is almost always a coercion situation. A druid has Charm Personed both heads in sequence (good luck), a cleric has bound the creature with a higher power's mark, or the party rescued one head from something worse and the other head grudgingly agreed to come along. Pick the framing that lets you keep the bickering, because the bickering is the whole personality.

In combat the ettin contributes 28 average damage per round through its Multiattack, which at AC 12 and 85 HP means it will trade hits with anything below CR 4 and lose to anything above. Aim it at the biggest enemy on the field and let the riders do work: Battleaxe to knock a target Prone for the rogue's Sneak Attack with advantage, Morningstar on the enemy spellcaster to spoil their next attack roll. The ettin will not switch targets without being told twice, once per head.

Out of combat the ettin is a logistical problem. It eats four rations a day, it cannot enter most buildings, it terrifies hirelings, and it has Darkvision 60 ft. but a Passive Perception of only 14, which means it is a poor scout. Its actual utility is doors. An ettin can break, hold, or pull anything the party cannot. Make the players use it for that, and let them watch the heads fight over which one gets to do the breaking.

Have one head develop a soft spot for one PC and the other head develop an active grudge against a different PC. Run every interaction through that filter. The party will spend more time managing the ettin than fighting with it, and they will love every minute.

An ettin is two stupid, angry brothers sharing a body and arguing about which of them gets to hit you first. Run it as comedy and threat at once. The heads bicker, point at different party members, claim credit for kills, and turn that energy into 14 average damage from each weapon.

Mechanically the ettin is a Multiattack mook with a twist: one Battleaxe and one Morningstar per round, both at +7 to hit for 14 damage each, with riders. Battleaxe knocks Large or smaller creatures Prone. Morningstar imposes Disadvantage on the target's next attack. Lead with Morningstar on the party's biggest hitter to soften their reply, then Battleaxe whoever has the lowest AC and watch the cleric eat the floor. Pinning two PCs in two different ways per round is the entire fight, and 85 HP at AC 12 means the ettin will be doing it for three or four rounds.

The condition immunity list does the heavy lifting against control. Blinded, Charmed, Deafened, Frightened, Stunned, and Unconscious all bounce off, because the second head wakes the first head up whenever something tries to switch one of them off. The bard's Hold Person fizzles. Banishment via a Wisdom save fizzles. Most fear effects fizzle. Tell the players that in narrative when their spell fails. "The left head's eyes glaze over and the right head slaps it." Make the immunity feel earned.

Ettins are not tactical. They will fight a wolf or a kobold or an adventurer with the same general approach, which is to hit the closest one and then argue about it. If you want a tactical encounter, pair the ettin with someone smarter who points it at things. A hobgoblin captain with one ettin on a leash is far more dangerous than two ettins together.

Have the heads disagree out loud about what to do with prisoners. The party who hears "eat them" answered with "no, ransom them, dummy" remembers that fight forever.