A pokemon becomes a Ash Pikachu when they are chosen by a pikachu pop star, and forms an inseparable team with that pikachu pop star.

During Night 1, each pikachu pop star must choose one pokemon to become their Ash Pikachu. The Ash Pikachu is added to the pikachu pop star’s secret channel. The Ash Pikachu and their pikachu pop star form an inseparable team; they will faint if the other pokemon faints. When this happens it is announced that the ash pikachus died.

The Ash Pikachu keeps their pokemon, however their base alignment is combined with the pikachu pop star's base alignment (usually wild pokemon). If the pikachu pop star and ash pikachu have different base alignments, they will only win when both of their base alignment's win conditions are fulfilled, forming a new alignment separate from either of their base alignments. For example, if the couple is pikachu pop star (wild pokemon base alignment) and a ratatta (team rocket base alignment), everyone else must faint (newly formed legendary pikachu pop star alignment). Or, if they’re both wild pokemon base alignment (e.g. pikachu pop star and bidoof), they both are part of the wild pokemon alignment and can win together with the other wild pokemon. The Ash Pikachu’s love overrides any other mechanic that may cause a pokemon to change alignment for any reason; the Ash Pikachu is always on the same alignment as their pikachu pop star.

This is a pokemon that is gained on top of your original pokemon - it does not replace it.