Character NFTs have rarity tiers (N, R, SR, SSR) and levels (1-50 per tier). Together Tier + Level can be called Character NFT Rank. For example, Normal.15. All other NFT types have only tiers.
Everything in SOULS OF MΞTΛ is an NFT. So you will earn XP for each character you play that will unlock new skills, and boost health and mana capacity. You may find more information on character tier upgrades in the Character NFT Fusion section of this Gamepaper. You may also read about Skill points distribution in the Leveling up characters section.
Each weapon or any other asset can be upgraded with SOM tokens to increase their tier and boost your attack power, speed, or any other relevant attribute. You may find more details about weapon upgrades in the Upgrading weapons section.
The higher the tier of your NFTs is the more powerful they are and the higher is their price on the Marketplace. So you have to progress through the game constantly to farm more tokens and upgrade your NFTs.