I was looking through Bulbapedia and noticed in Red’s sprite list there were sprites for Emerald, And for Brendan and May’s there are sprites for them in Firered and Leafgreen. What’s up with that? Was it used for link battles when they enter or what?
They're used for battling and trading. There's also sprites for every single Ruby/Sapphire trainer as well because the Japan games supported eReader trainer cards, so they had to keep the old trainer sprites for compatibility. They also do some extra stuff for compatibility, like including all Japanese fonts in the translated versions of Fire Red so the Pokemon will battle and trade properly, although the Japan version of Ruby/Sapphire renders English names in full caps monospace font, kinda ugly.
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Those sprites were used when you're able to see their Trainer Cards inside Cable Link Trades/Battles rooms. One player is playing RSE and another player is playing FRLG.
During Cable Link Battles you're able to see Red/Leaf (RSE) and Brendan/May (FRLG) sprites during the start and end of the battle.
