If you say the command word or hum the song you chose, a pink creature named Kirby appears. The rules for him are as follows:

1. It takes an Action to summon Kirby from the ring. He shares your initiative in combat and takes his turn immediately after yours. Kirby has movement and Action, but no Bonus Action or Reactions. Kirby does not count as an Ally to give you Advantage on attacks due to flanking. Kirby will stick around for 10 minutes with each summons (unless he is reduced to 0 hit points).
2. His hit points are 10 + 1D12 for each level of your character, which should be rolled each time you summon him
3. His AC is 10+ the characters proficiency modifier
4. His damage rolls are 1 + 1D4 for each level of your character.
5. His movement speed is 20ft and he has a 20ft jump (high jump and long jump) without a need for a running start.
6. His damage type can be Fire, Cold, Psychic, Force, Acid, Poison, Lightning, Thunder or Bludgeoning. You choose the damage time each time you summon him.
7. Kirby has proficiency (use the characters proficiency modifer) on Charisma, Wisdom, and Intelligence, and Constitution Saving Throws. Strength and Dexterity Saving Throws are +0.

When your character hits level 5, I think Kirby should be able to absorb spell scrolls and other items to gain temporary abilities.  At level 10, he would be able to absorb 2 spell scrolls to take on 2 different abilities. At level 15, he can swallow 3 scrolls. At level 20, he can swallow 4 spells scrolls. For example, if he swallows a scroll of invisibility, he might become Invisible Kirby. If he absorbs a scroll of fireball, he might become Flaming Kirby. I think having Kirby take on some properties of a spell scroll that he absorbs could be fun and allows you to find creative ways to use him. You can buy spell scrolls at enchanters' shops in most cities and the party has a wizard that would be able to create scrolls. I would use the rules from Xanathar's Guide to Everything for scribing spell scrolls.

Notes: By choosing a command word or song., Summoning