Throughout your journey, you'll encounter and gain the allegiance of several elemental Summon Spirits, such as Gnome and Efreet, as part of the main story. However, there are additional, optional Summon Spirits that can be found and contracted with through side quests. These include Undine (Water), Sylph (Wind), Volt (Lightning), Shadow (Dark), and Aska & Luna (Light). Each spirit is typically found in a specific dungeon or location, often guarded by a powerful boss.
Seeking out and defeating these optional spirits will add them to Mint's (or Arche's, depending on the spirit) summon roster, significantly increasing your party's magical offensive capabilities. For example, Undine is often found in a water-themed dungeon, while Sylph might be in a windy mountain pass. These side quests often involve solving environmental puzzles or navigating challenging areas. Collecting all Summon Spirits is a rewarding endeavor, as their powerful attacks can turn the tide of difficult battles and are essential for achieving 100% completion.