What a lot of people fail to realize is that in DMC1, Dante and Vergil had not seen each other since they were eight. Vergil had disappeared since that age, and only resurfaced when Dante was 28. That's why in DMC1 Dante has a flashback of him and Vergil as kids fighting over a piece of chocolate. Trish also said Dante had lost his brother to evil twenty years ago. Since he was 28 in DMC1, that means he lost Vergil at age 8.
DMC3 retconned it by making Dante and Vergil meet on the Temen-Ni-Gru at age 19. There Dante even said ''it's been a year since we last met''. So yeah, DMC3 was a retcon. That said, you could simply explain the flashback in DMC1 as 'one of Dante's fondest memories' of Vergil, and that they had indeed met when they were 18/19. Trish wouldn't have known about his meeting with Vergil on the Temen-Ni-Gru, because she didn't exist yet in DMC3.
Kamiya said it was his most important memory.
https://twitter.com/PG_kamiya/statuses/360298360252534785
you see this as a retcon and not a character development?
Yes