Frederick is a veteran who prefers to guide others, and is Chrom's deputy in the Shepherds, once serving as a royal knight beforehand.
Frederick is first introduced in the Prologue when Chrom and Lissa find the Avatar unconscious. He is a bit skeptical of their amnesia, but he eventually trusts Chrom's judgement in trusting the Avatar. Frederick accompanies Chrom throughout his journey in Ylisse to stop the Ylisse-Plegia war. When Emmeryn is nearly assassinated, he advises her to leave the capital for a short while. While they do so, Ylisstol is attacked by Gangrel, prompting Emmeryn to return to the capital to parley with Gangrel. Emmeryn orders Frederick to stay with Chrom, which he humbly accepts. After Emmeryn's sacrifice, he feels that he's failed his duty as a Knight of Ylisse, but he recovers and vows to continue to serve and protect Chrom and Lissa.