Sera

Sera is the daughter of Demon Lord Gustav, which makes her a demon princess. When the previous Hero, Serge Flore, invaded their castle, her father sealed her in a time-stasis spell and teleported her away to keep her safe, assigning Viktor to oversee and protect her. After Kelvin and his party defeated Viktor, Viktor entrusted her care to him, which is what leads to Sera joining Kelvin's party at the end of Vol 1 of the LN (and end of Vol 2 of the manga).

In Chapter 3 of Vol 4 of the LN, she officially joins Kelvin's harem as the second member (after Efil). In the manga, they are not depicted doing the deed, interrupted by Sabato bringing news of the declaration of war from Trycen.

Sera's main fighting style involves using her bare hands (thanks to the Combat Technique skill) clad in Black Magic debuff spells. She also has Blood Dominion, which enables her to control whatever and whoever it is that she gets her blood on, with one off-shoot ability being to suck magic out of spells through this skill. The second unique skill she acquires, Bloodbending, allows her to control her blood as she wishes.

Appearance
Sera has been described to be stunningly beautiful, with incredible proportions and bountiful breasts. As a demon, she also has a pair of horns on her head, a pair of wings behind her back, and a tail; however, all these are usually rendered invisible due to the Clip of Camouflage that she wears (although invisible, they are still present and can be used to fly/attack with/etc).

Sera prefers wearing military-style uniform, which she gleaned from Kelvin's memories and had Efil make. When working on golems with Kelvin, she also likes to put on a white lab coat. When fighting, she is equipped with Arondight, the pair of gauntlets that Kelvin made for her using Viktor's carapace.

Personality
Due to having been raised strictly within Gustav's castle without any contact with the outside world, there is a certain innocence and childishness to Sera's personality. However, she is also naturally bright and positive, and cares for Kelvin a great lot.

Her hobbies mainly revolve around fishing (which Kelvin taught her in Toraj and which she has displayed incredible talent with, according to one of the bonus stories of Vol 2 (LN)) and the playing of musical instruments (one of the bonus stories of Vol 3 (LN) depict her purchasing a piano).

She has been depicted as a "genius"-like character who does and gets things by feel and not logic, further compounded by her naturally sharp intuition and a plethora of detection skills. This has proven extremely helpful in activities like fishing (she can "sense" where the fish are), but makes her a terrible teacher (Kelvin attempted to have her mentor Miyabi as fellow users of Black Magic, to no avail).

Trivia

 * Sera's Evolved race of "Demon Blood Lord" does not make her a Demon Lord. She is a Blood Lord who is also a demon.
 * Sera is incredibly popular among the fishermen of Toraj for her prowess with fishing.