One more wild idea: I figured out how to make it all make sense, so the pairing is Byodoin
Told in third person
[Blurb]
I, Tsumu Teno, am a sixth-grade student at Hyotei Elementary. One day, a tiny person just ten centimeters tall suddenly appeared beside me.
He wore an unfamiliar red, white, and black uniform. His golden hair was a little long, and the stubble on his face made him look rather mature.
This figurine-like little man was no ordinary person. No one but me could see him.
He could float, hit a glowing tennis ball that demolished an entire wall, and kick me all the way to the center of the earth with his six-centimeter legs.
After he appeared, I never dared slack off during tennis practice or take matches lightly again, and my tennis skills improved by leaps and bounds.
Even my friends praised me for being so motivated that whole year.
Me: That's because there's a figurine-sized demon behind me!!
On my first day of middle school, after joining the tennis club, a whole scene unfolded.
Me: Hey, looks like something's happening over there. (Hinting frantically)
Byodoin: ...
Byodoin: You keep getting distracted and slacking off during practice. Are you trying to die?
So while everyone else rushed off to watch the commotion, I could only miserably finish the training routine he'd set for me.
Theme: Shake off laziness and refuse to lie flat
