Yakaminu is a miniature version of Yakamaru that can be unlocked as a pet with Yakamaru's helmet, which is obtained as a drop from killing Yakamaru.
After the item is used, Yakaminu can be summoned from the pets tab in the Hero interface.
Yakaminu has a drop chance of 1/300 with a 60 kill threshold. It is a requirement for the Insane Final Boss title. It is rewarded when killing the boss, rather than when finishing the raid.
After one of the dialogues, any interactions with the pet will refer to it as "Bertha" rather than "Yakaminu" temporarily.