Half-Dragon

A half-dragon ally is the bodyguard the party has been hired alongside, the dragonborn knight on loan from a temple, the bastard child of a wyrm who has decided this party is the safer option. Half-dragons are Neutral on the SRD line, which means motivation is whatever the GM wants. A clean frame is contractual: the half-dragon is being paid in coin or in a promised meeting with the parent dragon, and her loyalty extends to the contract's word, not its spirit.

In a fight on the party's side, the half-dragon is a frontliner with an emergency button. Lead with two Claws at +7 against the largest threat (26 average per round), and hold Dragon's Breath for the moment the party is bunched too close to fire safely on their own. The 30-foot cone will catch friendlies if she's careless, so warn the players the round before and give them a turn to spread out. Leap as a Bonus Action covers 30 feet, which means the half-dragon can be the one who jumps the gap to the enemy caster while the rogue is still climbing.

Outside combat she is not a friend. Half-dragons know they are exotic and they leverage it, charging premium rates and refusing menial work. She will not carry packs or babysit children, but she will translate Draconic, intimidate a guard captain, and stand watch on the dangerous shift. Pay her on time, address her by her chosen title, and never imply she is just a dragonborn with airs.

Have her name the parent dragon out loud once, halfway through the contract, and then never again. The party will write the name on their map.

A half-dragon is a CR 5 mid-tier brute that lets you put a draconic damage type into a fight without committing to a real dragon. Pick the Draconic Origin first (Acid, Cold, Fire, Lightning, or Poison), because every other choice in the encounter follows from it. A Fire half-dragon is a lava-cult lieutenant. A Poison half-dragon is the swamp tyrant. The damage type is the costume; the build underneath is the same.

The opener is Dragon's Breath if it's available: DC 14 Dex save in a 30-foot cone, 28 average damage on a fail (8d6 of the chosen type), Recharge 5-6. Save it for a turn when the party is clustered. Once it's spent, settle into Multiattack of two Claws at +7 with 10 ft. reach, 6 slashing plus 7 of the elemental type per hit. That's 26 average per round, and the 10 ft. reach means the half-dragon can hit two adjacent party members without moving. Leap as a Bonus Action covers 30 feet for 10 feet of movement, which closes range or breaks out of a flanking pin without provoking from anyone she isn't already engaged with.

Defensively the half-dragon is durable for CR 5: AC 18, 105 HP, resistance to her own damage type, and Blindsight 10 ft. The Blindsight only matters in 10 feet, so a Darkness or Fog Cloud wider than that still works. Wisdom and Dexterity saves are +5, but Constitution is +3 and Intelligence is +0. Hold Person, Banishment on a failed Charisma, or any Intelligence save spell will shut this fight down if the party has them ready.

Half-dragons are proud and rarely flee while a witness still lives. Once Bloodied, they Leap to higher ground, recharge breath if the dice cooperate, and try to take a PC down with them. If the breath stays cold, they fight on until 0 HP. The corpse is a trophy whichever cult, dragon, or warlord sent the half-dragon in the first place will want back.