Better than One. Yamato starts with one head, and gains more based on its constitution modifier.

Multiple Heads. While it has more than one head, Yamato has advantage on saving throws against being blinded, charmed, deafened, frightened,  and stunned

Whenever Yamato takes a percentage of its health or more in damage within a single turn (split among its number of heads and current health), one of its heads dies. If all its heads die, Yamato dies.

At the end of its turn, it grows one new head if any are missing. If Yamato takes fire damage, it must recover for one turn before being able to regrow its heads. Yamato regains 1d8+ bonus regeneration +[constitution modifier] hit points for each head regrown in this way, and can only regenerate up to its max limit of heads.

Reactive Heads. For each head that Yamato has beyond one, it gets an extra reaction that can be used only for opportunity attacks. When Yamato is able to increase the limit of its heads, roll a d4 and permanently add the result to your base health regeneration.

Constant Vigilance. While Yamato sleeps, at least one of its heads remains awake.

Deep Bond. Yamato has a deep connection with its master and friend, Haerrlekin. When she levels up, Yamato may increase one chosen ability score by 1. If Yamato should die, she may reduce her health to one to create Yamato's Egg.

Yamato's Egg. Once felled, Yamato will revert to this state so long as Haerrlekin invokes Deep Bond. Yamato will incubate for two days before hatching with one head and a portion of its stats, divided evenly amongst its maximum number of heads. Every dawn after, it will grow a new head, up to its max, and will restore its stats accordingly. While in this state, it will not recover in combat beyond its current limit of heads.

Multi Attack. Yamato attacks multiple times, determined by the number of heads it has available to it.

Bite. Melee Weapon Attack: +2 to hit, reach 10 ft., 1 target. Hit: 1 (1d4 +2 ) poison damage. At level 6, use d6 instead of d4. At level 11, use d8. At level 16, use d10

Breath Attack (Level 6). Ranged Attack: +2 to hit, range 60ft., cone. Hit: 1 ([number of same-element heads] d10) fire damage. Yamato must take a short rest before using a breath attack with any of the heads that have already done it.

Additional Action (Level 11). When Haerrlekin targets Yamato with a spell, Yamato may immediately take an action.

Victory Cry (Level 15). When Yamato or Haerrlekin deal the finishing blow to a target, they both regenerate, as if affected by Yamato's Multiple Heads trait.

Obliterate (Level 18). When Yamato is pushed into a corner, it can retaliate with great ferocity. Yamato spews a tiny orb, which travels up to 150ft. before exploding in a 20ft. radius for 1d20 per missing head fire damage. After using this, Yamato receives two stages of exhaustion, and must fully recover from it before it can use this skill again.

Tail Attack. Grapple Attack: +2 to hit, range 10ft., 1 target. Hit:1 (1d10 +2) piercing damage. At level 5, use 2d10. At level 11, use 3d10. At level 17, use 4d10. Upon being hit, the target must make a constitution saving throw. If they succeed, they take half damage and the barb detaches. If they fail, they take full damage and the barb holds true, leaving them unable to move, and giving them disadvantage on attack rolls. Every turn thereafter, they must make another constitution saving throw or take damage again. They may use an action to remove the barb from themselves, but must make a strength saving throw to do so. If they succeed, the barb will be removed from them and will painlessly detach from Yamato. Yamato can only use this once per long rest.

Protective Embrace. When an enemy attacks Haerrlekin, Yamato may use its own body to protect her, as a reaction. The next attack that would normally hit her will be intercepted by Yamato, attacking against its armor class and damaging it, instead of her.

Retaliate. When Yamato intercepts an attack for Haerrlekin, it may retaliate with a bite attack. If the attack lands, it may follow up with another bite, until it either has attacked with all available heads, or it misses.