Units can build support in almost every Fire Emblem game, and Three Hopes is obviously no exception. From the demo, pretty much every support pairing that was in Three Houses has returned, and we even get some brand new conversations for units like Monica. Unfortunately, not every support level has a conversation in Three Hopes. Some support levels between certain units only result in a support level increase which still provides a boost during combat, but does not yield a support conversation. There is also no evidence of S supporting in the demo.

Overall the system is still pretty much the same as Three Houses — build support by having units fight together and interact in the base camp in order gain some sort of combat benefit.

This page contains the following information.

The following things will increase support between units while in combat. Each action will only increase support once per battle, so there's no use in spamming.

The most efficient way to build support is to do it out-of-combat in the base camp. There are many activities that have units interact with each other, and these activities seem to give much more points than the amount given in combat. These are all the ways to build support between units in the base camp.

Shez can also go on Expeditions with allied units, which greatly increases support.

Units that have support built up with each other get bonuses when using the adjutant feature. In particular, units with support will gain the following benefits when paired, the higher the support level the greater the bonus.