Seru (セル) were initially benevolent creatures designed by God to exist symbiotically with humans, and help them to survive. However, when the Mist appeared they began to go crazy and attack humans.
When worn by a human, a Seru will grant the wearer great powers. Depending on the type of Seru equipped, these powers can include greatly enhanced strength, control over the elements, and even the ability to fly. With the strength of Seru, humans were able to build civilization and thrive on the bounty of their combined efforts.
When a Seru comes into contact with the Mist, it acts aggressively and will attack any humans in its vicinity. Furthermore, any Seru which comes into contact while already bound to a human will be able to take control of their host, effectively turning the human into a mindless monster. As long as the Mist pervades the air around them, the Seru and human cannot be separated without killing them both. Once the Mist has been removed, however, the Seru will return to its benign form, and the human will awaken unharmed. Humans freed from the influence of a controlling Seru often report that they feel as though they have awoken from a nightmare, and note that they have not aged in the time between enslavement and emancipation, an interval which can last anywhere from days to years.
All Seru possess an elemental affinity which determines what type of attacks it will use in battle (noted by the icon displayed next to its name). This is useful in determining what abilities can be assimilated by a Ra-Seru (see below).
Ra-Seru are Seru which were given special powers to govern Seru-Kai. When the mist enveloped Legaia, Tieg sent them to Legaia to help the humans against the Mist and its agents. They possess the ability to absorb power from the Genesis Trees to grow stronger, as well absorbing Elemental Serus.
Each main character eventually partners himself or herself with a Ra-Seru, who lends its strength and wisdom to the human. As the storyline progresses, both the main characters and the Ra-Seru come to trust each other, and the deepening relationship between them is reflected by the visible growth of the Ra-Seru following the successful activation of a Genesis tree. Once bound, a Ra-Seru strengthens its partner's primary arm for use in combat. When a human wearing a Ra-Seru defeats a Seru in battle, the Ra-Seru can absorb the Seru's power and later use it when necessary. Consistently calling upon that Seru's power will level it up, making it more powerful and causing additional effects.
Like most Seru encountered in the game, the Ra-Seru possess their own elemental affinities. Meta, Terra, and Ozma are associated with fire, wind, and thunder, respectively; these affinities can be seen by the nature of hyper arts executed with their power. For example, Vahn is capable of learning Tornado Flame with Meta, Noa and Terra can perform Frost Breath, as well as Gala being able to learn and use Thunder Punch (however, these moves require attaining special books found in chests hidden in dungeons or places covered by the mist).