Fusion is the mechanic by which you can fuse a base unit with any other unit. The base unit remains as-is but gains the fusion stats of the second unit, which is consumed/destroyed. One unit can fuse with a maximum of 30 other units.

In order to initiate a fusion, select the base unit you want to keep while in the camp menu, then click on fusion. Select the unit to be consumed in the process, and then confirm. You will lose the second unit but the base unit will have gained the second's fusion stats.

When you look at a character's status screen, there will be three columns: its base stats, its fusion stats, and its skill stats. The base stats are the unit's own stats, based on its class and level. The fusion stats are stats it gains from units that were fused to it, or if none were, the default that all units get (more on this in a bit). The skill stats are stats obtained through passive skills (like STR Up which gives you +3 strength at level 1).

Whenever you acquire a new unit, that unit will have default fusion stats so that if you fuse it with other units it actually provides a benefit. These are random, and determined by spending 3 points as such:

1 point can give 5 HP.
1 point can give 2 AP.
1 point can give 1 attribute point.

Level doesn't matter, and these stats will never increase on their own just by leveling. A unit might start with +15 HP, or +2 AP +1 STR +1 LUK, or +2 STR +1 MAG, etc. Either way it'll always be a total of 3 points spent using the above costs.

When a unit class changes, it gains 3 more points that are randomly spent.

However, a unit which you recruit when ALREADY class changed will not gain additional points. If you recruit a warrior, she will have 3 points. If you class change her to lancer, she will have 6 points. If you recruit a lancer directly, she will only have 3 points, and you've missed out on its class change bonus.

One unit can only fuse with 30 other units in total. This is the only limit.

It is worth noting that if you fuse a unit that has itself been fused with another before, it will consume multiple fusions, so it isn't possible to game the system by fusing a unit 30 times and then fusing that one to another.

For example:

Unit A is fused to Unit B. It consumes one fusion.
Unit C is fused to Unit B. It consumes one fusion.
Unit B is fused to Unit D. It consumes three fusions.

If you truly care about getting the most out of fusion (even if you play on hard, this shouldn't be necessary and it's a significant timesink), you will need to level up 30 characters who have access to 3 class changes instead of the usual 2. This will give you 30 characters who will provide 12 fusion stat points each, instead of the 3 from fusing spare recruited characters.

This essentially means that the end result character will have, instead of 90 fusion stat points, a whopping 360, which could result in extremely significant stat gains.

That's it for this guide. If you couldn't tell, Rime Berta is a very simple game, but very poorly documented in English. I do not, personally, suggest worrying about getting optimal fusion stats. The game is far too short to be worth that sort of grinding. Still, if you play on hard, knowing how to get the most out of fusion might help you out early on when those extra stat points can really weight in. Obviously, try to make sure you give the right characters the right stats. Your warrior doesn't benefit from MAG unless you're going the hybrid route, and your magician really doesn't need that STR.