In a sidequest, a backstory revealed that he and Alice were both orphans who lived in Hima. Decus was accused of robbing a few travelers, but it turned out that the real culprit was the owner of the orphanage and his three sons, but nobody believed him. When the travelers were about to attack him, monsters came in and destroyed the orphanage, burning the place down and killing many people inside. In the present, Decus revealed that Alice made a pact with a monster in order to save him and ever since then, he devoted his life into loving her. In the original game, it was revealed that the book was destroyed, thus destroying the pact with the monsters. It was revealed that only one person survived in the orphanage and moved to Iselia.
Tales of Symphonia: Dawn of the New World
One and a half years later, after coming in possession of Solum's core, Alice told Decus to use it to disguise himself as Lloyd and lead the Church of Martel in his name. In Palmacosta, he was the one that killed Emil's parents, which caused Emil to have revenge against the real Lloyd. He also used his disguise to attack Flanoir and took off his disguise in the church. He forced some bad-smelling colonge on Emil before leaving, thinking that he was a fan. Decus kidnapped Seles Wilder and took her to the Cape Fortress. There, he and Lloyd were fighting each other, but Zelos (and eventually, Emil) was able to figure out who the fake Lloyd was. After being (briefly) knocked out by Regal, he helped explained to Alice about the Centurion's Cores and how he wants to revive the Sylvarant Dynasty, which was destroyed 800 years ago.
At the Ginnungagap, he and Alice had their final battle against Emil and Marta. In the end, he died by protecting Alice from Emils fatal attack and amitting his true feelings for her.
Most of his artes were named after German words.