Himika Komatsu

Himika Komatsu (小松日海夏) is a character in Five. She's Itto's girlfriend.

Itto had been convincing Himika to give Yu-Gi-Oh a try. In an event/tournament, Koji, Itto and her will join as a team called something with "Metal", since they'll be the Metalmorph trio. Himika is not that interested in the game, but she's pretty smart and gets the hang of it, joining to support them.

The initial premise
Himika's Deck is a third Deck following the theme of Koji's and Itto's Decks. In contrast to their WIND and WATER themes, and to Metalmorphing with Spells and Traps, respectively, this one would be an EARTH Deck that Metalmorphs with monster effects. The "Metalmorph" monster is a tuner who can go into a Synchro during either player's turn, and the Synchros specifically require it as the tuner (similar to "Genex"), so there are lots of support cards for that specific monster.

It could include invulnerable effects, similar to the invulnerability in Soul Arena, to remain on the field. Example: "Once while this card is face-up, you can activate this effect (Quick Effect): This card's ATK/DEF become 0, you take no damage from battles involving this card, also, this card cannot be destroyed by battle and is unaffected by other card effects. These changes last until the next End Phase." It could feature creatures that protect themselves with their shells (like turtles, crustaceans, snails or armadillos). So, the Main Deck monsters would be EARTH Insects or EARTH Reptiles, and the Extra Deck monsters, EARTH Machines.

UPDATE: Another possibility for their playing style. They could practically "morph" into two different forms. They gain their first set of effects during your turn to attack, and the second set of effects during the opponent's turn to become invulnerable (hiding in their shells). The boss monsters could alternate between three or more effects.

Actually developing it
Himika uses an Iron-Shell Deck, the third Metalmorph Deck following the pattern of Koji's and Itto's. In contrast to Koji, who Metalmorphs Fusion monsters using Spells, and Itto, who Metalmorphs Xyz Monsters using Traps, Himika Metalmorphs Synchro monsters using tuner monsters.

Her monsters have a defensive mode which they go into during the opponent's turn to make them remain on the field, hiding in their impregnable shells.