Floch Forster is introduced as a former recruit of the Survey Corps who joined the regiment shortly before the mission to retake Wall Maria's Shiganshina District. He was assigned to Squad Klaus and, alongside Hange Zoë and Squad Levi, became one of the few survivors of the ensuing battle. In appearance, Floch is of medium height with short, tousled hair. Over the course of the story, his hairstyle changes, with his bangs growing thicker and swept to the right side.

Initially, Floch openly admits to being a coward, with a deep-seated fear that his death should not be meaningless. This fear is prominently displayed during the Battle of Shiganshina District, where he panics under the assault of the Beast Titan. Despite this self-proclaimed cowardice, he proves capable of courageous action, willingly participating in Commander Erwin Smith's suicide charge and later physically restraining Eren Yeager from interfering with the injection of the Titan serum. A dominant trait in his early personality is his profound respect for Erwin Smith. He argues passionately that only a "demon" like Erwin could lead humanity to victory, and he is openly critical of Captain Levi's decision to save Armin Arlert instead of the Commander. He views this choice as an emotional and childish one, in contrast to what he sees as the logical and necessary decision to prioritize Erwin. He is also brutally honest and insensitive, telling Hitch that Marlowe likely regretted his final choices and informing Armin that many in the military believe his survival over Erwin was a mistake. His traumatic experiences lead him to insist that future recruits be fully informed of the horrors they face, so that "cowards" like him would never join.

Following the revelation of humanity existing outside the Walls, Floch's personality hardens significantly, developing a strong hatred for the outside world and adopting a vindictive and radical nationalist ideology. He becomes a fervent follower of Eren Yeager and a key member of the Yeagerist faction, ultimately serving as its de facto leader. He is dedicated to the establishment of a "New Eldian Empire" and believes that Eren is the only one capable of saving Paradis Island through a full-scale Rumbling. In this role, his actions become increasingly ruthless and devoid of empathy. He is open to scorched-earth tactics, such as burning civilian areas in Liberio to deny the enemy resources, and he shows no concern for collateral damage. He does not hesitate to use violence against anyone who is not an Eldian or who resists Eren's efforts, including former allies in the Anti-Marleyan Volunteers and even his old comrades in the Survey Corps, whom he perceives as traitors. He harbors a deep and driving fear of retaliation from the outside world, genuinely believing that Eren's death would lead to the genocide of all Paradisians. This fear fuels his extreme actions and determination.

Throughout the story, Floch's key relationships are defined by his ideology. He greatly admires Erwin Smith as a necessary "demon" and later transfers this admiration to Eren Yeager, seeing him as the new force to save Eldia. He is antagonistic toward Armin Arlert, viewing him as an unworthy replacement for Erwin. Although he is allied with characters like Mikasa Ackerman and Jean Kirstein initially, he ultimately brands them as traitors when they oppose Eren. His development is a clear trajectory from a frightened, insecure recruit into a fanatical and zealous leader. He fully embraces his role as a necessary villain for the sake of his people, even as his methods become extreme and his worldview narrows.

In terms of notable abilities, Floch is not depicted as an exceptionally skilled soldier in combat compared to the Survey Corps' elite, but he possesses significant strengths in other areas. His most prominent ability is his indomitable will and determination. After the Battle of Shiganshina, he develops a powerful survival instinct and the resolve to see his objectives through, no matter the cost. He demonstrates keen intellect and strategic cunning. He devises the plot to assassinate Premier Zachary, orchestrates a military coup to give the Yeagerists control over Paradis, and uses his experience as a Shiganshina survivor to rally followers and manipulate public opinion through selective leaks of information. He is also a capable user of vertical maneuvering equipment, with years of experience allowing him to navigate combat situations effectively. During the final battle, he shows incredible tenacity, clinging to a ship for two days while injured and making a final desperate attempt to destroy the alliance's only means of stopping Eren.