Seshima Rui

On 11 October, Rui debuted her first cover of the song "Kimagure Romantic."

On 25 October, Rui reached 10,000 subscribers on her YouTube channel.

On 11 December, she revealed on her Twitter account that she would get her 3D model, but not with her standard outfit, instead with an idol outfit.

On 19 December, Rui debuted her 3D idol costume during the live: "774Fes Do It!"

On 22 February, she reached 20,000 subscribers on her YouTube channel.

On 3 April, Rui officially debuted her 3D model on her YouTube channel using her main costume.

On 4 May, Rui debuted her second 2D costume. In this costume Rui got a new hairstyle using long twintails, she uses white and blue triangular earrings, a white choker and a necklace. Rui wears a white blouse with blue neck, bottom and her surname "Seshima" accompanied by a dog face to the right, blue shorts that are torn and she wears a red belt, she uses asymmetric brown socks with the left one being longer than the right one, her shoes are blue on the top and heel is dark blue. Her fingernails are painted yellow on her right hand Rui uses two wristbands the first one is yellow and the other one is dark blue and carries a white and dark blue cap with two yellow lines.

On 10 August, she held a stream to celebrate reaching 30,000 subscribers on her YouTube channel.

On 19 December, Rui debuted her third 2D costume. In this costume Rui got a new hairstyle using small twintails tied with black bows. Rui wears a nurse inspired white dress with red lines as decorations around the neck, the laced portion of the sleeves and under the chest, the white bottoms of her dress also have crossed red lines and the edge of her skirt also a wide outline in this color. The sleeves of her dress are mostly see through, at first sight the skirt seems to have an opening to the right side however the skirt also has see through cloth in this area decorated with a black ribbon, in the left side, the skirt takes the shape of a heart, the opening make her black stocking belt more visible, the belt keeps in place the white sock on her left leg, however even if she wears a sock Rui has a black band-aid on top of it, on her right leg she wears a leg belt in her thigh, just below it she has another black band-aid, a small black sock and Rui wears black shoes. Rui also has the option to wear a white nurse cap with a small red heart.

On 21 March, the VTuber groups "AniMare," "Honey Strap," "Sugar Lyric," "Hiyoku no Crosspiece" and solo talents would be merged into one group, including management, aiming to make it easier for the talents to thrive under just one brand. this meant that Rui was now merged into the main branch.

On 7 March, it was announced that Rui would be one of five 774 Inc. members to participate in the Life Like a Live! 7 Idol show. she would be performing on the Saturday solo.

* On 15 July 2021, 774inc C99's Twitter account announced that its talents would appear in the event Nanashiinku bake~shon! in which every member would appear in limited time merchandise including original artwork by studio A-1 Pictures depicting them in swimsuits. The merchandise included, posters, hand fans, cushions, keychains, pins, beach balls and special commemorative drinks associated with each group. The event was held in the EJ Anime Theater in Shinjuku City from 22 July to 1 August.

* Rui Channel / 瀬島るい 【あにまーれ】 - YouTube channel
* 774inc. - AniMare's members page
* @Rui_Seshima - Twitter account
* @rui_seshima - Marshmallow account

* #生ぬるーい - Stream
* #きりとるい - Clips
* #あとるい絵 - Fan art

* Seshima Rui - Social Blade YouTube stats
* Seshima Rui - Playboard YouTube stats