Decks
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Utopic Variant
Leaning into its ability to put multiple Rank 4 and 8 xyz monsters on the field, Galaxy can field extra formidable boards with Rank-Up-Magic and Utopia's ZS archetype. Number 99: Utopia Dragonrar summons multiple Number monsters from the Extra deck, and access to Number F0: Utopic Draco Future is consistent. The addition of Photon Delta Wing is a great boon to this variant.
Horus Variant
As a Rank 8 engine that does not consume the normal summon, Horus is a natural partner for Galaxy. King's Sarcophagus and the Horus monsters allow making Number 90: Galaxy-Eyes Photon Lord earlier in combos, and potentially aid in follow-up the next turn - just mind Photon Lord's materials. Numbers Last Hope is a strong inclusion here, so feel free to splash additional Number monsters in your toolbox.
Therion Variant
Therion monsters are popular additions to archetypes making Rank 4s or 8s. With Galaxy Soldier and Photon Orbital as targets for Therion "King" Regulus - the latter of which can perform its search after being equipped - Galaxy has innate synergy as well. This variant breaks Galaxy Trance lock more often than pure, and prioritizes Gigantic "Champion" Sargas and Springans Merrymaker routes instead.