Since her moving into Mustafau is one of the main divergence points from canon, Ochako appears during the Prologue arc.
Itsuka Kendo appears in at the end of Chapter 3 and is formally introduced in Chapter 4, well before the Entrance Exam arc, while in canon she is introduced alongside Class 1-B at the Sports Festival. Likewise, her classmates, here her best friends, Setsuna Tokage and Yui Kodai, also appear, along with a cameo from Eijiro Kirishima.
Nighteye makes an earlier appearance during the training sessions at Dagobah beach, to sway All Might's opinion on who should be his successor.
Gran Torino gets brought to one of Kendou's training sessions by Nighteye, for him to compare her to Mirio Togata.
Tenko Shimura is introduced during at Chapter 7, before the USJ.
Melissa Shield is introduced at Chapter 9. The events of Two Heroes are set after the Final Exams arc.
Nejire Hado and Yuyu Haya appear in the end of Chapter 9. Shoto Todoroki, Mina Ashido, Ibara Shiozaki, Mezo Shoji and Hitoshi Shinso also make cameos.
Himiko Toga debuts in Chapter 10, during the Entrance Exam.
Nine and his crew are introduced in Chapter 13. In canon, or at least if you consider the movies canon, he is introduced after the Meta Liberation Army arc.
The High End Nomu Hood is brought by the Consortium for the USJ Invasion. In canon, it debuts during the Pro Hero arc.
Endeavor, Miruko, Edgeshot and Tensei Iida show up to help drive the Consortium's thugs back during the USJ arc, whereas Endeavor and Tensei weren't introduced until one arc later at the sports festival, Edgeshot first appearing during the Hideout Raid Arc, and Miruko showing up during the Pro Hero arc.
Shin Nemoto and Hekiji Tengai of the Shie Hassaikai appear on chapter 18 for a meeting with Teko Kendo.
Tamaki Amajiki and Mirio Togata appear before the Sports Festival to help train 1-A.
Camie Utsushimi, Eri and Overhaul appear on chapter 22, before the Sports Festival.
Ryukyu and Chitose Kizuki/Curious, Rikiya Yotsubashi/Re-Destro debut during the Sports Festival.
Lady Nagant debuts during a flashback over a month before the Sports Festival.
Gentle (Vigilante), La Brava and Ippan Josei all debut during the Sports Festival, nearly an entire year before their respective canon first appearances.
Katsuma and Mahoro Shimano make cameos during the Sports Festival.
Flect Turn and Rody Soul debut during the Sports Festival, over an year before the Humarise arc, although under the disguise of original characters Daikoku Yaoyorozu and Skip, respectively.
Rock Lock, Fat Gum, Uwabami, Selkie, Sirius, Rappa Kendo, Joi Irinaka, Hari Kurono, Rikiya Katsukame, Trumpet and Moonfish all debut in Chapter 34, right after the Sports Festival arc. Skeptic, Geten, Hawks, Beros and Magne debut in the following chapter.
Chikuchi Togeike, Ms. Joke and Wolfram all debut during Chapter 38, right before the Internships arc.
Retroactive example: Saiko Intelli, whose name here is Kanshi Shiryoku, was originally introduced during the License Exam arc. Here she first appears during a flashback between the USJ and Sports Festival arc.
Maina Furasu, Support Course student introduced during the Final War arc is introduced shortly before the Internships arc.
Instead of debuting during her namesake arc, Star and Stripe first appears during Internships arc.
Seiji Shishikura and Inasa Yoarashi, originally introduced during the License Exam arc debut during the Internships week.
Dictator who first appeared during Dark Hero arc, alongside Kunieda and Gashly Eijiju who debuted during Final War arc, all make their first appearance during Internships arc.
