Fuse

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During normal times, you can Fuse unneeded companions into higher tier companions at the following rate: 
* 8 Novices -> 1 Expert
* 8 Experts -> 1 Elite
* 8 Elites -> 1 Master
* 4 Master -> 1 Master
For special event rates, see: Fuse-Up Event.
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These are general guidelines that have higher priority over the Should you fuse? table.
#Generally advised never to fuse Masters : you never know if they might get reworked and turn into Gods. Even if the list below says you can fuse it, it's best to keep at least one copy before fusing the rest.
#F2Ps should keep the following to get to +10: 
##one or two Elite DPS of their choice (Kantes and Loujin are popular; Relicpaiton is good for shields initially but don't +10),
## two or three elite SK (Kentom > Heatgrill), 
##and the healers Kino >> Foria > Anaris. You can fuse the rest of combat elites.
#Don't fuse Explorers (apart from the few lck/dsc based novices and experts) until you have at least one +10 copy, especially of the area-specific jobs. 
#Never fuse Station Companions of Expert or Above.
#Save Expert Combatants in hope of Fuse-Up Event.
#Fuse away any Novice Combatants (except Stacia or something if you're making them +10 in accordance to the Beginner Guide): they take too much space to save them over months for the event.
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The following table is meant to resolve your questions about fusing companions, and how many +10 should you create before freely fusing the units. 
You should first follow the General Guidelines, then come here, for example, if you want to fuse a Master regardless of future change, and want to know if it's currently safe to do so.
Use CTRL+F.
Companion 	Fuse?	Reasoning
Aries 	No 	Amazing tank and can't be knockbacked. Lots of uses.
Jin Andre 	No 	Amazing tank, knocks enemies back, heals hinmself. Amazing.
Lutes 	No 	Unique shield-grant pattern, useful for alliance bosses and other things.
Jin Anaris 	No 	Just Anaris with better stats and fantasy.
Poisonous Seyona 	No 	Just a Seyona with more damage less knockback/stun, with high stats and fantasy.
Antenore 	No 	The most powerful single target mage right now, big threat in both PvE and PvP, very versatile.
Ashes 	No 	Used mostly for DEF buff and AoE reduction.
Celenia 	No 	Average healing, crazy buffs. HP, MATK resist, crit, she has everything.
Elise 	No 	Fragile, but a powerhouse, especially in cube fights where she deals massive damage thanks to being the only MATK adventurer with 4x multihit and her HP% evaporate skill being especially potent against high HP targets. Also she cute.
Elle 	No 	Absolutely crazy healing, MATK buff is a bonus. Sometimes used against lower level alliance bosses.
Emily 	No 	Very strong anti-archer PvP unit, with high damage output and stacking DEF buff and some AoE reduction. Her single target damage is a bit lacking, but she is still a viable choice for Klaudteron alliance boss due to her buffs.
Jinryong 	No 	Despite starting the combat in short range, he is still considered range, shields add to his overall survivality, and he deals decent damage. Great pick for all alliance bosses.
Juhong 	No 	Absolute killer in PvE due to very high fire damage, still viable in PvP, but with mage meta got a bit weaker, especially on higher ranks.
Karan 	No 	Very solid adventurer with team ASPD stacking buff and ok damage, evade adds some survival. Useful in Ashwood alliance fight, but only at lv1 with only 3rd skill unlocked (can't have evade)
Lin 	No 	Known for her ASPD/MSPD buff, places shields on allies which is a very good skills against Ashwood and to certain degree Klaudteron. In PvP debuffs enemy crit.
Nohara 	No 	Very strong in PvP, some great PvE uses with fantasy - makes blue and purple cube bosses much easier.
Sylar 	No 	The ultimate boss killer (higher damage output compared to same and even higher upgrade Antenore), but rather weak against normal mobs, and useless in PvP.
Altran 	Yes 	Suprisingly high damage, but limited usability due to no shields/multihit and other similar skills.
Alpha 	No 	Fantasy buffed, his death self-destruct can wipe enemy team. Almost no healing, puts barriers instead, debuffs enemies.
Aria 	Yes 	Slow, low single target damage, her only possible use would be as Nohara's replacement on purple cube boss (same attack range), but without MATK reduction she will receive much more damage.
Arley 	No 	Reworked into a goddess. Fantasy Arley is amazing, but mythic works just fine.
Ash 	No 	She's great for some niche situations. Limited CC and healing reduction can see some use in PvP, but she isn't particularly strong.
Asmodeo 	No 	One of the strongest adventurers. Strong low-rank PvP adventurer (with yet untested potential for high-rank PvP) but rarely seen in PvE content due to lack of shields.
Bek 	Yes 	Extremely bad Healer. Weaker Anaris.
Bold 	Yes 	Bad damage.
Brocan 	No 	Fast and with penetration block, but not that good outside of that.
Bronks 	Yes 	Very slow with weak skills.
Dylan 	Yes 	No good skills. Some shields make him a bit useful against boss if there is absolutely nothing else available. Mass pull can have some funny uses.
Edin 	No 	Fallen titan. Jin Andre is objectively better.
Ein 	No 	He was a mage-hate unit before it was cool, but also before they were any good. Somewhat useful in arena, but not a priority at all.
Eida 	No 	In PvE used pretty much solely for her last skill which removes buffs and shields, she makes certain rift stages (notably 70 shields) a total joke. Potential to mess with mages in PvP. Also used in events to remove Boss's 50% HP buff.
Elim 	Yes 	Very low single target damage, but she is great for Natrix boss shields removal.
Eline 	No 	Her stacking MATK resist buff gives hope for interesting tactics in the future.
Elpaiton 	No 	A bit low damage but shields and multihit along with Knockback reduction make him pretty useful. Fantasy Elpaiton is popular as well.
Han 	No 	No good skills.
Heidi 	No 	Multihit, some shields, anti-monster. Limited use but great damage in the right case.
Hon 	Yes 	No good skills
Jeanne 	Yes 	RIP 2018-2018. Died from patch.
Gift Bundle Kuiyung 	Yes 	DEF reductions, and knockback on adventurer might be useful one day, but overall didn't show much promise so far.
Jin Bachmaul 	No 	Good anti-archer skills, team wide AoE reduction buff, buffs Erner Mora tribe, interesting choice for anything with strong archer mobs/boss
Lawrence 	No 	Synergy with Antenore and Elise. Skills not exactly the best, but do something, which can't be said about all SKs
Layaul 	No 	Flixdog has the same skillset is the best monster tank, Layaul sees less use only because it's harder to get him high enough. Can equip gear so is better in some cases.
Lipy 	No 	Healing debuff, healer stuns, additional damage against healers, and if healers aren't present (which is rare), still doing at least ok damage. High upgrade Lipy can get absurdly strong, and even low upgrade ones are responsible for killing off the last standing HP setups in PvP.
Marinaton 	Yes 	No good skills.
Marine 	No 	Basically worse Lipy.
Moomyung 	No 	No good skills, but high upgrade fantasy makes him viable.
Naias 	No 	Weaker and more fragile Emily.
Paiton 	No 	Shields and base AoE hits.
Penqueen 	Yes 	No good skills.
Randowl 	No 	High crit rate (with lowerd crit damage to compensate) combined with very high ASPD and fast skills and multihits, saw some sparse and surprising use in PvP, and possibly can see some use in PvE.
Reny 	No 	Combines some buffs from Celenia and Lin, and offers multi target healing, but at the end of the day you just want to use the other two.
Ronya 	No 	Decent AoE damage.
Rod 	Yes 	(much) Worse Layaul.
Rudra 	No 	Good CC but no knockback and low AoE damage. With fantasy, she is an arena butcherer.
Seyona 	No 	Knockback, AoE damage got a bit better after DEF nerf.
Shamu 	No 	No good skills.
Taron 	No 	Strong low-rank PvP adventurer (with yet untested potential for high-rank PvP) but rarely seen in PvE content.
Urlat Bach 	Yes 	No good skills.
Vernum Grendel 	Yes 	Lost all value after damage reflect nerf.
Vita 	Yes 	Out of season April Joke.
Wayne Heights 	No 	Very low damage, but a great support unit in PvP with 30% HP buff being used sometimes in PvE for HP wins.
Winter Festival Kantes	Yes 	No good skills.
Winter Heatgrill 	Yes 	No good skills.
Yunho 	No 	Multihit and evade. The definition of an average adventurer.
Cayan 	No 	6x multihit OP. Does a lot of damage.
Jin Elina 	No 	Great Aoe damage, has burst aoe at battle start.
Patis 	No 	5x Multi-Hit Archer. Elim who?
Sweety 	No 	Still pretty bad at Fantasy+10. (Rip Dracobringer)
Jin Andre 	No 	Basically a balanced version of pre-Nerf Edin. A very useful tank at fantasy.
Anaris 	No 	Used for knockback and stun resistance buffs irregularly.
Belfling 	Yes 	No good skills.
Celestring 	Yes 	No good skills.
Fledin 	Yes 	No good skills.
Foria 	No 	Great healer for domain battles and Fire cube boss.
Heatgrill 	No 	Great tank when fire damage comes into play.
Kantes 	No 	Good damage.
Kentom 	No 	Simple but great Shield Knight.
Kino 	No 	One of the best Elites.
Len 	Yes 	No good skills.
Lilimton 	Yes 	No good skills.
Loujin 	No 	Very high damage adv.
Lulu 	Yes 	
Muhyul 	Yes 	Buff isn't worth giving the spot to him.
Orin 	Yes 	Buff isn't worth giving the spot to him.
Relicpaiton 	No 	Decent early on for shields.
Sigyn 	Yes 	No good skills, insanely slow.
Totte 	Yes 	Buff not worth giving spot.
Uranus 	Yes 	Not a very popular choice for Elite SK.
Yona 	Yes 	Bad healer, has decent buffs.
Yuma 	Yes 	High MSPD, some people +10 her..