Undead Fortitude. If damage reduces the sidekick to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the sidekick drops to 1 hit point instead.

Improved Critical. The sidekick's attack rolls score a critical hit on a roll of 19 or 20 on the d20.

Proficiencies. All Armor

Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: (1d6 + 1) bludgeoning damage.

Second Wind. The sidekick can use a bonus action on its turn to regain hit points equal to 1d10+5. Once it uses this feature, it must finish a short or long rest before it can use it again.

Defender. The sidekick can use its reaction to impose disadvantage on attack roll of a creature within 5 feet of it whose target isn't the sidekick, provided the sidekick can see the attacker.