One of the most important objectives in Digimon Survive is to complete the long list of critters you will discover along your journey. Although early on this might be straightforward enough by simply recruiting monsters to your team, many of the games' more powerful monsters are more reliably collected by evolving them from lower level creatures.
How to evolve Digimon
Depending on which type of Digimon you are, there are two strategies to develop them. Digimon that are bound to characters and follow you through the main story evolve at specific times during your adventure, usually during battles and accompanied by cutscenes; higher levels of evolution cost more SP at the start of each turn. Agumon, specifically, has an even more intricate evolution system.
While playing Free Battles, the Digimon you befriend during the negotiation works a bit differently. Higher level evolutions require higher rarities of slabs, so be careful with your usage. These are consumables and they arent reversible, so you may want to recruit more than one of the less evolved Digimon to complete the game's roster.