SSQ - Sequential

Overview
SSQ - Sequential is a character that shows in Terrasteel's video Terraria - AFK Dungeon Guardian Kill Glitch.

SSQ - Sequential was a small YouTuber that made Terraria content. He peaked at 11 subscribers before deleting his channel on August 25th, 2018.

His Socialblade profile is still available, and it can be seen here.

Trivia
Photon's channel was created 5 days after SSQ - Sequential's channel deletion on August 30th.

SSQ - Sequential's top channel video was showing how to defeat the Lunatic Cultist, a idea that would be recycled by Terrasteel 17 months later.

Photon is proven to be SSQ - Sequential. A fun fact about this is that Stardust Armor has the same color scheme as the Photon orb (yellow and blue).

SSQ - Sequential has a constant intro catchphrase: "Hello arrow clickers, my name is SSQ - Sequential, and today...".

The arrow clicker greeting has been parodied into broken Chinese, reading "您好箭敲，我的名字是SSQ - Sequential，和今天我会玩Terraria" [Nín hǎo jiàn qiāo, wǒ de míngzì shì SSQ-Sequential, hé jīntiān wǒ huì wán Terraria] . This translates literally to "Hello arrow [knock], my name is SSQ - Sequential, and today I will play Terraria".

Link Between Photon and SSQ - Sequential
In Terrasteel's Moon Lord Arena and Weapons Guide, a character with the name "pingping2993" was hosting the world. Normally, SSQ - Sequential would host the world. Later in the video, Terrasteel can be spotted killing the Moon Lord with SSQ - Sequential, wearing his iconic Stardust Armor livery.

For a few frames at 1:37 in the video, Terrasteel's mouse flicks over the stardust armor player, and the name of the player makes out to be pingping2993. This confirms SSQ - Sequential to be Photon.

This means that Photon had a YouTube channel before his current channel.