Rachnera is an attractive young woman with short lavender hair with bangs covering the right side of her face. She has six pupil-less, monochromatic red eyes and her teeth are all fangs. Her most noticeable physical trait is that the lower half of her body is that of a massive spider, with a large skull design on the back of her spider abdomen.
She has black carapace covering her arms from the shoulders down, and gauntlet-like plating on her hands that end in sharpened points. Her fingers are also very long.
Her attire consists of a halter top that reveals a good deal of the lower half of her large breasts and her entire waist, shoulderless sleeves, and a shirt, all black with a gold and slightly-frilly white trim.
Most of Rachenra's personality seems to be determined by her bad experiences with Humans. Because of this, she initially hated Humans, and believed them all to be hypocrites. Rachnera was convinced that, at heart, Humans were mean-spirited creatures and that any form of kindness was simply a "mask" they wore. While she believed this to be true of Kimihito as well, she was shocked to find out his kindness was genuine. This changed her outlook significantly, though it did not completely change her. Despite this, Rachnera is still a cynical person at heart, such as when she believed that, as a male, Kimihito would inevitably cheat on his homestays.
One of the biggest impacts her past had on her is that Rachnera greatly dislikes any form of dishonesty. No matter how ugly the truth is, Rachnera would still prefer it over pleasant lies. This is demonstrated when she managed to goad Cerea into admitting she hates her. Rachnera was actually happy with this, despite Cerea standing up for her to Miia earlier. Rachnera told Cerea that if she dislikes her (Rachnera) than she should do so openly. One way to make her genuinely angry is to mock her history and motivations.
She seems to have somewhat sadistic nature, judging how she delighted in wrapping her webbing around her former host. She also enjoyed torturing Lilith in sexually suggestive ways after Lilith broke down Rachnera's personality and history and mocked it. In a much minor degree, Rachnera also enjoys riling people up, frequently purposefully saying the right thing to make them angry. She has a very easy target with both Miia and Centorea, due to their mutual dislike for her.
Of all the homestays, Rachnera is the most openly perverted, and the one who enjoys (accidental) perversity on Kimhito's part the most obviously. From what is shown, Rachnera greatly enjoys bondage, getting a great kick from tying her victims up in her own webbing, both sexually and non-sexually. She is also a skilled Dominatrix, as she was capable of turning Lilith into her personal sex toy. She also mentioned that she considers both Kimihito and Cerea her "toys". However, dealing with both Papi and Suu while the rest was away proved to be too exhausting, even for her. She is also revealed to be somewhat obsessive about perfecting her bondage technique, which annoys most of the other monster girls (except Suu, who cannot be bound, and Mero, who rather shamefully enjoys it) to varying degrees, though she seems to have exempted Kimihito from being webbed up in this fashion. However, she seems to include every monster girl who lives in Kimihito's home or is a possible rival as fair game, a fact that has Lala more than a little scared of her.
She is implied to be somewhat lazy, and can frequently be seen slacking off.
However, even despite her sinister traits, it is clear Rachnera's the most mature of all the homestays, and can quickly discard her playful attitude and become very serious if the situation calls for it. Ironically, despite being built like a dangerous predator, Rachnera has great self-restraint, and Kimihito even noted that she's the only member of the household who has never genuinly hurt or endangered him, even by accident.
Despite all this, according to Lilith, Rachnera's personality is merely a façade she puts up. Rachnera pretends to be uncaring, while in reality she is afraid of being rejected again for her features, and that she is very self-conscious about her appearance. Rachnera's angry reaction implies that there is some truth to those words.
It's unknown how much Rachnera's personality differed before her bad experiences, if at all, but considering she seemed to wear more conservative clothing back then it suggests she may have changed a lot.