For those not familiar with F/SN, all you need to know for this topic is this: seven "Masters" summon seven heroic spirits in seven self-explanatory classes. Those classes are Saber, Archer, Lancer, Caster, Rider, Berserker, and Assassin.
Of course, Henedge could fit the Saber class, but what about the others? And yes, I realize that there's seven classes but only six Pokémon in a team. I'll switch up the team every now and then so they all see some action.
"My favorite color of poop is seven." - Seanbaby, cracked.com The Official Nox of the Kingdom Hearts 3 Board
Archer could be a Pokémon that could be part of a branched evolution line considering in the storyline he's
the protagonist from the future, and he (Archer) wants him to take a different path than he did. The Pokémon could also symbolize the different choices that were available for the two, and how these choices made Archer who he was.
A bit symbolism heavy, but it's a thought.
You know you read too much when you dream about flying books.