What Masters to keep multiple copies of?

Written on the 9th of September, 2020.
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I usually stumble upon this question for new players, especially early on when the game throws so many stuffs at you and you're afraid of making mistake. So here we are, I'll list down some notable Masters that you should keep multiple copies instead of upgrading them.
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For explorers, early on, you will want at least 2 copies of each explorer (More if it's Dane, Morgan or Effie), with 1 copy being upgraded and the other ones being left at +0.
Why?
Early on, you're really lack of good explorers, thus having more Master explorers to cover the explorations is recommended. Furthermore, later on, when you have a decent amount of explorers, you can still migrate these +0 copies to domain to boost your coin earning, so nothing is wasted here.
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Stationers are split into 2 categories:
- Town Stationers: Hertin, Serakan, Raina, Parencho (The last 2 are event and selector ticket only)
- Dungeon Stationers: The rest
For Town Stationers, it's best to only have 1 highest upgrade copy instead of multiple ones, with the exception of Raina, who is still decently good at +0e36 Fantasy (better than most Novice/Expert/Elite Town Stationer)
For Dungeon Stationers, the situation is the same as explorers, 1 highest upgrade copy and 1 +0 copy is recommended. All of them are good.
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Here comes the most wanted part. Combatants.
Well, this is also the most complicated part. The list can be changed base on the new contents being introduced. I'll list down the current notable combatants that you should hold multiple of:
* Taron: Exactly 2 copies, for Raid's Claw/Body
* Sylar, Hecate, Ash: These are boss killer units, are extremely useful for Raid, as well as alliance bosses.
* Seyona: Now this is a controversial one. I personally don't like having 2 of her, since something that can be solved by having 2 Seyona can already be solved with just 1. She's not as strong as other boss killers, so multiple of her in bosses is not as good as multiple of the above boss killers. However, having 2 of her is an extreme advantage early on in PVP/PVE, and becomes less useful in the later stage of the game.
Honorable mentions:
* Bronks: Reworked Bronks is a powerhouse, PVE/PVP/PVB. He is the highest scaling physical damage dealer without 6th skill. So if a situation where multiple adventurers can be used and survive, he's the best choice.
* Lutes: His 4th skills can stack up on a single unit, providing that unit with an insane amount of shields. Can be niche in certain situations.