SOLUTION: MOON OF STARSANSWER: SHU ITSUKI
This puzzle is a series of five "criss-crosses", with the first four depicting an image of an anime boy at its intersection. Reverse image searching these images reveals that these anime boys are characters from the Ensemble Stars!! gacha games. In order, those characters are Adonis Otogari, Wataru Hibiki, Hajime Shino, and Ritsu Sakuma. Their names don't fit into the across or down entries of their criss-crosses, but the key to filling them out is the addition of the words MAIN and SHUFFLE above the criss-crosses.
Ensemble Stars!! divides its characters into several groups called units. This is a term from the Japanese/Korean pop idol industries, where large pop groups and agencies will have performances by a small group of their performers, who perform under their own name as a unit. In Ensemble Stars!!, every character is part of one "main unit", but promotional events over the game's history have introduced temporary "shuffle units", whose members are assembled from completely different main units as a crossover. Each pictured character's main unit fits into the across slot of its criss-cross, and one of their shuffle units fits into the down slot. (Notably, non-alphanumeric characters are also included, and words and special characters in unit names are separated with bold borders.) The units are:
Adonis Otogari: UNDEAD (main), Butou-Kai (shuffle)
Wataru Hibiki: fine (main), Puffy☆Bunny (shuffle)
Hajime Shino: Ra*bits (main), EVIL NUM+ (shuffle)
Ritsu Sakuma: Knights (main), La Mort (shuffle)
(As an easter egg, the cells represented by the character portraits spell UNIT in the given order.)
The letters with Roman numerals in their cells can be written into the matching cells in the final criss-cross, with a nondescript silhouette at its intersection. The entries across and down are VA_KYRIE and B_END+, which can be completed with an L to spell the units Valkyrie and BLEND+. The only character in both of those units is this puzzle's answer, SHU ITSUKI.
Some wild coincidences in setting this puzzle. First: with the limited number of units in Ensemble Stars!!, I got really lucky that the shuffle unit for the answer, BLEND+, was not the only unit in the game with a + in its name. (That + is one of the only special characters to appear in more than one unit name, so this was super fortunate for me.) Second: I did not intend for the image cells to correspond with the letters in UNIT! That was a total accident, and it was after noticing this that I decided to use that as the given ordering. Third, and most disappointing: I didn't find out until my partner finished solving this puzzle that Shu Itsuki is actually their most hated character in this game; from their perspective though, it is fitting that Shu is represented by an L.