Fusion is where two of the same fighters are combined to make a stronger version. Fusing two regular fighters together will result in a fighter+, which is stronger. If that fighter+ has another fighter fused into it, the fighter will turn into a fighter++ which is even stronger than before. If you fuse a fighter++ with another fighter the rarity of the fighter will go up as shown in the table below. The fighter on the left hand side will be the fighter of which 100% of stats is carried on to the new version. The fighter on the right hand side will have a percentage of its stats pushed into the new version. An exception to this rule is when the rarity of the fighter on the right is higher than the fighter on the left (for example, left-hand-side fighter=SR+ and right-hand-side fighter=SR++).
Fusing at Max Level
If two maxed out (level 20) Nerio's are fused, they will result in a C+. If two level one Nerio's are fused, they will also result in a C+, but it will be much lower in stats. These 4 Nerio's, two maxed and two at level 1, were fused. When they are fused both the Nerio from the maxed fighters and the Nerio from the unlevelled fighters both began as a C+ at level 1, but the stats were greatly different. As you can see in the 6th picture, the game warns you when fusing two unlevelled fighters that the result will be very bad compared to if you fused levelled fighters. Notice the stat difference between pictures 3 and 5. It is also shown in a mission where you have to fuse two fighters together, and Whyson explains that 2 maxed fighters will have a better outcome than that of 2 unlevelled fighters. It is often said that the golden rule of Rage of the Immortals is to fuse fighters only when both copies are fully maxed.
Fusing unlevelled fighters
Fusing maxed out fighters
Both Nerio's are C and are both completely unlevelled.
Results in very low stats, barely changed from the original C copy.
Both Nerio's are C and are both at level 30 (maximum level)
Results in much higher stats than fusing two unlevelled C's.
Methods of Fusing
There are generally two main methods of fusing, the 4-fighter way and the 8-fighter way. The 8-fighter way will result in the maximum possible stats for that fighter, although it is often a very lengthy, expensive, and time consuming way of levelling your fighters. The 4-fighter way is reccomended for new players and lower rarity fighters as it is cheaper and the stat difference between the 4-fighter method and the 8-fighter method is not great for lower rarity fighters. Other methods include the 6-fighter way, and the quickfuse (not reccomended).
A total of 4 identical fighters is needed, although this method will not result in maximum stats.
A total of 8 identical fighters is needed, this method is the only way to achieve maximum stats.