VoloKari refers to the romantic relationship between two characters from the Pokemon franchise.

Volo is from Pokemon Legends: Arceus. He is the antagonist of the game.
There is a popular fan theory that Volo is a time traveler. For this particular AU, he is given a new name.
This belief comes from Pokemon Platinum where

Akari is the female player character and protagonist from Pokemon Legends: Arceus.
In fandom, Akari is seen as the same character as Hikari, the female protagonist from Pokemon Diamond/Pearl/Platinum, or as related to her in some way.
Some view Akari as an ancestor to Hikari, some view them as siblings, it is all dependent on the AU (Alternate Universe).
To make matters more complicated, there is also another character named Dawn who is from the Pokemon anime series.
Some people choose to view her as being the same as Hikari (that is her Japanese name). Others still view all three characters as separate individuals.
It really is up to you!