Kaoru is the jinchūriki of Son Gokū — the Four-Tails. Not much is know about Kaoru and his tailed beast except that it had ability to mix its earth and fire natures, to create lava which it spewed from its mouth. Due to being its jinchūriki, he also possessed this ability.
Kaoru has displayed the ability to enter a "Version 2" form, by moulding its chakra into a human-like shape around himself. When entering into this form, the sudden increase in chakra was such that it instantly vaporised.
In addition to forming a dark chakra cloak around his entire body, he also gained prominent tufts of fur along his back and the characteristic horns of Son Gokū on his forehead. With the power granted by this form, Kaoru not only manifests all of his beast's four tails, but also becomes able to summon even larger torrents of lava than normal almost instantaneously.
Kaoru has demonstrated the ability to manifest one of the beast's tails. In this form he can envelop himself in an armour made of lava. The extreme heat generated by his armour is strong enough to burn his opponent, even without coming in direct contact with them.
Full Four-Tails Form
While fighting Tsubasa Yuki and seeing how strong he was and his skills are Kaoru had no choice to go Full Four-Tails Form. In this form, he takes on the appearance of Son Gokū itself, allowing him to manipulate the full extent of the beast's considerable physical strength.