Back on Tetromino, I don't get the "it's not a character" argument against Tetromino because like... that's the entire point. I think most people who want Tetromino are at least partially motivated by the desire to play as a weird faceless object with no personality - I'd absolutely say that's my reasoning at least.
Also, on Arle vs. Tetromino: there's no reason these characters can't coexist, and unless they decided to split up 5 slots for each Tetrominos or make a true stance fighter, Tetromino would likely be the quickest and easiest non-clone imaginable. The animations would be simpler than Steve, they wouldn't summon any props except for clones of their pieces and maybe existing 2D game assets, and the moveset would likely be among the most simple since the HAL era. The biggest difficulty would be animation clarity, which - not a game dev, so don't take my word on this - seems like more of a creative issue and not a technical/logistical one. Arle has actual abilities, and her own RPGs, there's not much reason for her to have a block-stacking moveset, and even if there was, her moveset would have no overlap with Tetromino on virtue of 1. not being a block herself, and 2. the differences in shapes between the two games' block structures.