How To Raise Your Skill Level
Every Digimon has a main, sub, and passive skill. Out of these 3, only the main skill can be leveled up. Raising the skill level has the effect of increasing the effectiveness of that skill, making it more powerful. Note that the most you can level up a skill is to level 10; this is contingent on the Digimon generation. If your Digimon hasn’t evolved then the maximum cap of their main skill will be lower than 10; you must evolve the Digimon in order to raise the skill level to the max.
Beyond what’s stated above, you can take advantage of a Digimon’s individual traits and use them to your advantage. By taking Digimon belonging to the same evolutionary lineage, you can fuse them together to raise the Base Digimon’s skill level.
You can fuse them by selecting a base Digimon and selecting the ‘Up Skill Level’ option. After doing so, you will be shown a list of potential Digimon you can fuse with – whatever Digimon you fuse with will disappear, so be careful during the selection process. Furthermore, fusion requires you to spend Bits (in-game currency). In fact, you can use Bits alone to raise your Digimon’s skill level. You’ll be given an efficient way of collecting and earning Bits later in this very guide.
A few things to note in relation to fusion: You cannot fuse with a Digimon who belongs to a different generation (if a Digimon has evolved then you cannot merge with them); main skill and sub-skills will change with fusion but not the passive skill. Finally, if one of the Digimon you can fuse with is part of a team (presumably set as a team member or Partner) then you cannot merge with them.