Could it be the Ragtime Mouse?

I am nothing like my brothers,
not a single little bit.
This would be a reference to the Agares enemy in Ipsen's castle, who are physically similar enough to be called his "brothers".

My partner is gone -
he's moved on for good.
The Agares enemy always appears alongside a Gargoyle, but not the Ragtime Mouse.

Now you must consult others,
to know what I should
be able to tell you,
but I could say no more.
The answers to his quiz questions are found throughout the game.

A failed experiment,
an incomplete archive of lore.
His questions, along with the correct answers that you find out elsewhere, make for some interesting trivia (or "lore") about the game world.