The Heroine's Mecha-ude is an Overprotactive Parental Figure
One potential explanation for why it resides in her skirt is so whenever any creep tries to get their way with her, it then comes out to strike with a multi-punch while going "THIS IS FOR MESSING WITH HIGH SCHOOL GIRLS!"- Jossed. Sinis and Dex act more like loyal servants to Aki.
There is more than one Trigger Arm out there.
It's typical anime progression. Hikaru and Arma's Trigger Arm Super Mode seems to be the only real power-up in the entire story, and as such, if the villains are to remain being a threat and other good guys are to stay relevant, their Mecha-Ude partners can't just constantly be getting stronger and stronger themselves. There will likely be reveals of more Trigger Arms out there, one of them possibly being Fist (as he and Arma are brothers) and another most definitely being an antagonist because not having a Trigger Arm duel would be a massive wasted opportunity.