In return, let me tag along wherever you're headed. You see, I... I've lost my memory. The only thing I know is my name...and nothing else.
Desch is a wandering vagabond, who joins the party as the third party member. Compared to the other guests, Desch's time with the group is the longest, with three dungeons and a couple of towns.
The party first hears about Desch from a lovesick woman named Salina and her subsequently angry mother in Canaan. This leads them to climb Mount Jenora, also known as the Dragon's Peak.
The party actually meets Desch in Bahamut's nest, where he initially teases them for being caught, but owns up when they point out that he did as well. Desch explains that he actually lost his memory and knows that he's supposed to do something but cannot remember what. Desch and the Warriors of Light decide to become partners and set out to figure out Desch's destiny.
Eventually, the party comes to Gulgan Gulch where the Gulgans tell Desch that his destiny awaits in the Tower of Owen, before giving the party the Toad spell so that they can enter. As the party climbs the tower, Desch begins to regain his memories and realizes that he is connected to the tower.
At the top, the party confronts Medusa, who overloads the reactor of the tower; This is bad as the tower is responsible for keeping the Floating Continent in the sky. To repair the reactor, Desch tosses himself inside and warps the party out of the tower, seemingly to his demise...
...Or so it seemed. When Doga sets out to find people to save the Warriors of Light, Doga finds no one in the tower. However, Desch cimbs up nearby, having just finished fixing the reator. Doga brings him to the Crystal Tower. In the end, Desch returns to Canaan and is reunited with Salina.
Desch is generally a light-hearted guy, joking around and talking casually. After getting the Enterprise, he makes a comment about how the party can go to exotic ports and meet exotic women, although the party's response leads him to insist that he's kidding. In the end, he does go back to his girlfriend Salina, fulfilling his promise to return to her.
Upon reaching the Great Library of Saronia, the party can find two volumes of the Book of Owen, written by the man who created the tower. In the second volume, it is revealed that Owen is Desch's father, and that he placed Desch into a cryogenic sleep once the Flood of Light occured in order to protect the tower. It's overall implied that the lengthy sleep played a role in Desch's initial amnesia and that the Great Earthquake was responsible for him waking up.
The remake gave Desch a number of changes and touchups, starting with his design; Gone were the purple onesie, brown hair and pads on the shoulder. In their place was a purple coat, blue-green hair and a strange device with a lens to allude to the original pads.
The expanded script also led to some differences. A number of Desch's interactions are principlly with Refia; Usually his playful nature plays off of her low tolerance for foolishness, especially given Desch's desire to handle business first before seeing Salina, though she does feel sympathetic to Desch's memory problems.
Some of Desch's interactions are also expanded upon. Most notably, speaking to Desch after speaking to the people of the Village of the Ancients causes his memory to start resurfacing, resulting in one of the first mentions of the first half of the game taking place on a floating continent. The remake also makes it a bit more clear that Desch gets more serious upon realizing the gravity of his duties.
Of course, unused text is where the real meat is...
Gameplaywise, Desch has both a physical attack (using his signature sword) and Thundara. As a bit of foresight from the developers, during the portions where the party is mini, Desch will prioritize Thundara due to physical attacks being ineffective.
The Books of Owen were rewritten, leading to a few differences: First, the events of the game's backstory were shuffled around and unused text implies that Desch was active during the Flood of Light. Desch is also suggested in Owen's third and final book to be an inventor in his own right, explaining how he's the one to maintain the tower.
A lot of interactions between Desch and the Warriors of Light were parred down. Some of these interactions include Desch telling Arc that he was even shorter when he was Arc's age to make him feel better about being mini (a claim that Ingus finds suspicious), Desch putting on a brave face for the party when they enter the Vikings' Cove only for Refia to spook him, some tomfoolery between him and Luneth after they retrieve the Nepto Dragon's eye and a discussion on Japanese pronouns which seemingly plays a role in Ingus using "ore" for the rest of the game.
Most interestingly, Desch's Village of the Ancients scene goes a bit into the nature of the Flood of Light, suggesting that the sun stopped moving. This is one of several hints about the nature of the Flood of Light in the unused text.
Desch also has a number of non-text related content linked to him. While Desch does wield his sword in-battle, it can be hacked into the inventory (although it cannot be equipped). There are three possibilities regarding this, from most likely to least likely:
Additionally, there is an unused room in the Tower of Owen that is exclusive to the remake. It's not a dungeon-like room; Instead it's a bedroom with a few amenities such as a desk and a drawer. Dialogue indicates that this room is where Desch was left in cryosleep during his time between Owen finishing the tower and the beginning of the game.