You know how every French boy in anime has the same sparkly personality (ex. tamaki suho, yuga aoyama) I hc mettaton(ex) to have that personality very dramatic and flashy, he also talks like them (i never rlly liked jacksepticeyes voice for him)
As for mettaons original form (box thingy) he has a more robotic voice (the voice jack used for him, maybe a bit lower) but is still flashy and dramatic.
Mettaton is definitely asexual. He's aromantic but he definitely act flirtatious towards EVERYONE I dont think he would want a serious romantic relationship. There is a handful of people he deeply cares about tho. (alphys, frisk, etc.)
