She ties her hair in a ponytail and also wears a pair of glasses.
She seems rather protective of Miseng and they both come from the same family and are of the same generation level. This is because they share the same surname "Yeo" (여) and they share the same generation name "Seng" (생): 미생 (MiSeng) and 고생 (GoSeng).
SIU has confirmed they're not sisters, insinuating they're most likely from the same family and know each other very well (probably cousins).
Yeo Goseng was the eighth person to enter the testing room. Ja Wangnan was about to attack her guiltlessly because she was not wearing a uniform, but was stopped when Miseng ran to her, calling her unni. Kang Horyang's wing turned into hearts when he first looked at her. While they were waiting for the next person to come in, Wangnan started to yell at everyone for being so compliant. When he shouted at Miseng, Goseng screamed back at him and hugged Miseng. When the next competitor, Hon Akraptor, tried to vote Miseng off, Goseng yelled at him as well to stop doing whatever he wanted. A fight broke out between Akraptor, Horyang, and Jyu Viole Grace, when Viole announced that he would drop everyone. After Viole blasted the two of them away with shinsu, Goseng identified the mark he wore on his coat as those worn by Slayers in FUG. This shocked everyone and caused them to fear Viole. She was hugging Miseng after Viole defeated Horyang and Akraptor.
While they were waiting for the second part of the test to begin, Miseng gave everyone (except Akraptor) a chocolate bar ("chips" in The Company's translation). Wangnan angrily asked, "What do you think this is, a picnic?!" to which Goseng smacks him and told him to just eat it. She scolded Miseng when chips were given to Viole, telling her that she shouldn't do dangerous things. When another Regular made it out the elevator, bloody and weak, she stood up to help him but was stopped by Wangnan, who said that they would have to drop him later anyway.
Goseng scored worse than Miseng for the World's Greatest Shinsu Contest. She could be seen sitting against the wall along with the other Regulars who thought they had failed. When Wangnan started to cry as well, she scolded him and told him to be manlier and that his tears were making Miseng cry too. Faced with both of their weepings, she gave up in trying to stop their tears. She is surprised by Wangnan's attempt to convince Viole to pick him. For reasons unknown, she never appealed to Viole to select her.
She encountered Akraptor and Horyang after they came out of the Baths. At that time, Miseng was crying. Then, Ja Wangnan and Nya Nia appeared with a pack of cards and enticed the others to play. After the game, she expressed her wish that all of them just stay on that Floor and live out their content lives.
She and Miseng took part in the game initiated by Mule Love. After instructing Miseng to never open the door except for her, she took the initiative by recruiting Wangnan. She had a short conversation with Wangnan, telling him how she came in expecting to lose, but seeing Wangnan's constant determination to pass, she felt inspired to do the same. Later on, she guarded a room while Wangnan, Akraptor and Horyang stole rooms. After the test, she was seen celebrating with the other passing Regulars.
On the 21st Floor, she stayed with the Regulars she passed with. She was living with them in the house Viole bought for them. When Viole brought home his fellow FUG members, she cut the fruit they brought as a gift (though she trembled in fear while doing so).
On the 21st Floor test, the team was split into two groups. Goseng went with Miseng, Wangnan and Akraptor to search for the Flower of Zygaena that they were sent to retrieve. She used the "Box" on her Lighthouse to search for information. When they saw a pig-like creature which had the flower on its back, she slowed it down with her Lighthouse. They chased the pig to a hole and, though she protested at first, she let Miseng go in to look for it. While Wangnan and Akraptor went to look around, she stayed behind and supported Miseng with her Lighthouse. She got Prince's message to help Viole as he fought Urek Mazino and she arrived just in time to see Urek chase after Viole. However, she and the rest of the team could do nothing to stop Urek from reaching Viole.
On the 28th Floor, she, Miseng and Prince used their Lighthouses to hold down the enemies until Yihwa used her flames to defeat them. Afterwards, she found a picture of Cassano on the Box and showed it to Horyang. He was shocked and told everyone that this was the Devil of the Right Arm.
After Miseng and Prince were asleep, she along with the rest of the team listened to Horyang's story. Afterwards, she stayed up waiting for Horyang to come back but fell asleep outside in the living room. Viole covered her with a blanket, then left.
She and sat with the others as Akraptor explained the situation. She also sat by as Wangnan accepted Khun Aguero Agnis into their team. Over the ensuing month, she and the rest of the team trained relentlessly under Khun. Though she had improved, she still lacked confidence in her abilities, especially now that Viole and Horyang were gone. However, these worries proved to be groundless and she along with the rest of the team passed the 29th Floor test.
She and the others were kept waiting due to Wangnan's oversleeping. As they passed into the 30th Floor, she looked awestruck at the enormity of the Archimedes. As their Suspendiship landed, she and the rest went into their dorms, fighting about their rooms. Later, she and the rest of Team Tangsooyook, except for Wangnan, went out because they were bored. They went to the lighthouse (the kind that emits light to guide ships, not the Lighthouse Light Bearers use) but left Miseng and Prince at the bottom because the two of them were afraid of heights.
She told Khun that Miseng and Prince had been left behind because they were scared of heights when Khun contacted her. They then waited a while to avoid the initial rush, but when they opened the door they were confronted by Chang Blarode. Akraptor was about to be shot when Wangnan and Khun turned up and made Chang drop his gun. Khun told them to go on Archimedes, but she protested, thinking of Miseng and Prince. However, when Akraptor backed Khun up, she then relented as well and was sent up.
She was selected for the fourth group of the "Plug" event. At the moment of her opening the door, she was startled by the fights between Regulars; fortunately, Horyang protected her from the attack of another Regular. Chang who noticed Horyang affection towards Goseng then commented on their relationship in battle. Chang then told them to avoid Varagarv.
At the after-party, two disguisedpeople brought Horyang to her and the two proceeded to dance. Horyang told her that he'd met Sophia again, but everything he knew may have been a lie to further their research.
She alerted Viole, Khun and Rak when Horyang wasn't waking up; she only told them and Wangnan because she didn't want the others to worry. She was present when Sophia and Beta diagnosed his condition being due to his Devil part being stolen (by Cassano). She volunteered herself to stay behind with Horyang, whilst the rest of the team climbed the Tower.
She was very briefly seen sitting next to a comatose Horyang with her face buried in her arms.
Powers and Abilities
Goseng is a Light Bearer. She is sharp and knowledgeable and was the only person to notice that Viole was wearing the mark of the Slayer from the organisation of FUG. She is one of the more mature members of the group and helps calm them at times.
Centripetal Pressure Control: By using 4 Lighthouses, Goseng can stop the movement of many enemies at once. It was used to trap weak Regulars and then finished them all with one blow. One Lighthouse will be placed at the center while 3 Lighthouses will surround the central Lighthouse.
Notes and Trivia
Goseng's glasses are so round and big that they cover half of her face. We never see her eyes. It's unknown for now if she wears them because she has poor eyesight or because they aid her in battle, like Po Bidau Gustang's glasses.
Her name "Yeogoseng" literally means "female high school student". Goseng can also mean "suffering, hardship".
Like Miseng, there's a running joke about her being Enryu: Gosenryu.
(To Wangnan, Horyang, and Akraptor) "It's fun to hang out like this. Wouldn't it be good to get out of the Tower and live like this all together? And completely forget about the Tower."
(To Wangnan) "I wasn't confident. I got this far by being lucky, but now I know that they are so many people better than me. I got depressed, I came here to take my last test. I thought 'I would fail anyway'. I left my house wearing slippers. I just signed up to get it over with. I had no confidence in myself. But then, I met you and Miseng. And somehow I passed the test. You are as desperate as I am. Watching you fight, it made me change. I want to trust myself a bit more. Just like you."