Tenacity has a cuteness
My own, probably a mated pair of watchman gobies: very protective of each other. One fainted during tank emergency and move: mate stayed by and kept propping her guy up until he came to.
Runner up: tailspot blenny and starry blenny.
I can only say buy them as a pair or don't try: sex is not that obvious, even when you know; but the fish sure do. I brought a mirror in view of my clearly male bright yellow watchman and the poor guy nearly killed himself trying to kill 'that fish.' The duller pair of watchmen I had were blue-edged, rather plain fish, and definitely a pair, arrived as a pair, and lived happily for years: lost one due, I think, to age, and the other followed in fairly short order.