The Yosawa Siblings

The Yosawa (四沢) Siblings are a group of characters in Five.

They are three siblings who will serve as secondary characters in an event. These characters are a reference to Yu-Gi-Oh! GX's Daichi Misawa, each using a Deck based on his different element Decks. Furthermore, since Misawa's Decks appear to reference different areas of science, these characters could be from a family of engineers specializing in different areas, reflected by their Decks.

Saburou
The younger sister, Saburou (三郎), is a Chemical Engineer undergraduate, the only of the siblings who's still in college.

Reflecting her chemical area, Saburou uses Misawa's Water Deck, a Deck that puts together monsters representing the different chemical elements to Summon stronger monsters.

Jirou
The middle brother, Jirou (二郎), is an Electrical Engineer.

He uses Misawa's Earth Deck, featuring "Magnet Warriors" that manipulate the opponent's moves by toying with attraction and repulsion.

Besides being able to repel enemy's attacks and targeting effects, the user can also force the opponent to target their own monsters with each other's effects by granting them opposite poles, or even force them to battle against each other with effects like "Battle Attraction".

Ichirou
The eldest brother, Ichirou (一郎) is a Mechanical Engineer. He uses Misawa's Fire Deck, which wasn't shown in the series.

This Deck consists in quickly Summoning "Fire Dragon" and inflicting heavy damage, turning the opponent's monsters into FIRE Pyro monsters that damage their own user.

Theme-wise, the Deck is based on the Otto Cycle of a four-stroke internal combustion engine, following the pattern of Misawa's Decks (Water being chemical, Earth being electrical, and Fire being mechanical). The name "C³" means "Combustion Cycle Critter".

As a variant, the Deck may also be used alongside Saburou's "Water Dragon" for a Water & Fire Deck, since both dragons have synergy with one another. It's a less consistent variant, but with a deadly combination that can inflict heavy damage very quickly mixing both Dragon's effects. In a tournament, this character could run a full Side Deck which, when swapped around, turns the Deck into this variant. He uses this variant against opponents he judges that should be defeated ASAP, with this variant easily delivering OTKs.

Alternatively, it could also be featured in a Tag Duel where Saburou and Ichirou duel together.

Possibility: The parent
Possible idea: There could be a fourth member of the family with the WIND Deck, also based on yet another branch of science, who is actually Nexus' mysterious Wind Admin. This could possibly be their mother, making these siblings still be relevant later on when the Wind Admin enters the scene.

Their Deck could be based on more advanced science, like astrophysics, and revolve around complex combos that function with a gimmick similar to "Macro Cosmos", only, the owner's cards return to their hand when banished. The boss monsters could also revolve around complex subjects such as quantum physics, 4th dimension or alternative realities.

In this case, to go further with the theme, she could have two subordinates using the Light and Dark Decks.
 * The Light Deck could play with the concept of the duality of light - about how it works as both a wave and a particle, depending on what it interacts with. It could feature dual monsters that alternate between two "configurations" depending on the situation. It could feature Synchros, with part of the "different configuration" also being becoming tuner or non-tuner.
 * The Dark Deck could be themed around dark matter or dark holes. The latter could feature monsters that "absorb" other monsters, gaining the effects of other cards. It could feature boss Xyz Monsters that, during the End Phase, absorb all other monsters on the field into itself as Xyz Materials.