Anivoice allows Koji to communicate with and command animals through speech.
Koji seems to be able to understand animal language or otherwise receive information from the animals he commands. This makes Koji a versatile hero due to the diversity of abilities found within the animal kingdom. For example, bugs with their small size and burrowing capabilities are able to perform sneak attacks from underground, birds are useful for reconnaissance due to being flying creatures with great eyesight, and so on.
In the manga, Anivoice is portrayed through a unique speech bubble, with undulated borders.
Animals must be within Koji's vocal range in order to be influenced by Anivoice, otherwise it won't work. Humans, despite being animals as well, are exempt from the effects of Anivoice. This Quirk is also useless against artificial creatures that have been shaped to resemble real animals.
Anivoice is sound-based and, because of that, any noises louder than it (such as other sound-type Quirks like Voice) are likely to disrupt its effects, either by drowning out Koji's own voice or scaring away nearby animals.
Koji is always polite with animals when requesting for their assistance, with all his pleas having been met so far. Koji appears to have a great relationship with them, which might be a possible effect of his Quirk, as well.
Sometime before the Provisional Hero License Exam, Koji improved his Hero Costume with a voice-amplifying device, which he wears over his jaw. With this piece of equipment, the sound range of Anivoice greatly expands, allowing Koji to lead a large quantity of animals at once.
Koji is afraid of insects and other bugs, which somewhat hinders his communication with them. Despite that, Anivoice works with those animals just as well as any other.