Mr. Dyslexic Heart Bear is a dog plushy that Stephen owns, that appeared on the vlog occasionally during Stephen's college days. Its name is a combination of Mr. Bear Rug from Day 274 and the fact that the dog "ripped out someone's heart that's tattooed [from] a man who didn't know much of the alphabet" (Stephen, Day 195), since Stephen used the same voice for both the bear rug and the dog.
Nevertheless, it was not known as Mr. Dyslexic Heart Bear until Day 330, when Stephen received a letter from Conneticut with a picture of the dog plushy, from an anonymous author who referred to themself only as Mr. Dyslexic Heart Bear. With the letter, it also sent a floppy disk that had "Learning to love means Learning to live" written on it. On Day 376, Stephen found out that the floppy disk contained a children's word game on it, possibly referencing its dyslexia.
After that first letter, Stephen received a few more letters from Mr. Dyslexic Heart Bear on Day 475 and Day 554. In all the letters, Mr. Dyslexic Heart Bear expressed its 'love' for Stephen and disappointment that Stephen did not return its feelings. However, in the last letter that was sent towards the end of Stephen's college life, Mr. Dyslexic Heart Bear announced that he would be "return[ing] to a regular bear again," which suggests that that was the last letter sent by the original Mr. Dyslexic Heart Bear.
Since then, Stephen has sometimes shown the plushy remnant of Mr. Dyslexic Heart Bear on camera, like on Day 568, or referenced it, like on Day 985 and Day 2588. Several viewers have also sent mail in to Stephen and Mal, posing as or referencing Mr. Dyslexic Heart Bear, like in StephenMail #18, #44, #60, Dec 2013, Mar 2016 and Jun 2016.