This animation isn’t really finished, but it showcases his abilities well enough for it to be posted:
Mellow has two main abilities, one from the Mask and one from the Device, but for now, the Mask is what we’ll be focusing on. The Mask’s power is called Jester’s Masquerade, and it has two subpowers called Warlock’s Eye and Phantom’s Box.

Warlock’s Eye allows Mellow to either stop time altogether with a significant boost in perception, reaction time and general speed at the cost of the amount of time allowed to be stopped OR merely slow down time with a minor boost to perception, reaction time and general speed. If his entire body is used for Warlock’s Eye (100% WE) then time is stopped altogether for only 10 seconds. However, if a part of his body is used, like his legs or arms (≤50% WE), then time is only slowed for a max of 30 seconds. This can be used for punch barrages, rapid fire kicks, and faster uses of melee weapons. Thinking is a free action as cognitive speed is also boosted substantially, allowing him to plan his best course of action as long as he pleases. Repeated use of 100% Warlock’s Eye within 15 second halves the allotted time every time it is abused. Same goes for ≤50% Warlock’s Eye. Also, if Mellow loses focus during use of any kind of WE (like getting hurt just a little or being distracted too much) then WE fails and is cut off.

Phantom’s Box is the other subpower of Jester’s Masquerade, and allows Mellow to phase out of existence for an incredibly brief time. When out of existence, he is immune to damage and is able to pass through objects. But the same way that people can’t hurt him goes both ways as he can’t hurt people either. Momentum is also a very important factor when using PB, as there is no gravity in non-existence, nor is there any real way to direct yourself without propulsion. This is why his boots are equipped with a hover function, both for transportation and for moving in this state. Phantom’s Box can be seen as a double-edged sword, as using it just once leaves him perfectly fine. But if he uses it again before 10 seconds have passed after the first use, he will feel much more tired. Repeated use will make him pass out due to exhaustion. Phantom’s Box in general is used to confuse enemies, dodge attacks and escape responsibilities.

TL;DR: Mellow is a DIO knockoff