There are few characters in Genshin Impact that has pets or summoned units that actually help your team to deal more damage.
I will list down the characters that might be a viable addition to your team.
1) Xiangling - with Gouba in her side, Gouba will eat a spicy chili which will make him breathe fire dealing Pyro Damage. After Gouba's duration expires, he will leave a chili that will increase the character's ATK% by 10%.
2) Fischl - summoning the mighty Oz. Fischl can be swapped to other characters on your team after she summons Oz. Oz can deal tons of Electro damage based on Fischl's ATK.
3) Sayu - using her ninja arts, she can summon the Muji-Muji Daruma that heals the current character and will attack enemies once the character reaches its 70% HP. The Muji-Muji Daruma deals Anemo damage which is good for swirl reactions.- summoning the cute Baron Bunny that deals Pyro Damage. Baron Bunny can also get the enemy's Agro which can help you to re-engage the enemies. You can remotely detonate the Baron Bunny if Amber reaches C2. Who do you think is the most viable addition to your team?