It seems that the sudden reveal for Nilah was due to the last champion, Bel’Veth, receiving a lot of leaks prior to her official reveal. The near-instantaneous reveal for her could avoid the appearance of more leaks, which could drastically affect the hype for new champions. Not to say that Nilah didn’t receive leaks of her own, but compared to the previous release, it was far less in number.
The trailer showcased both fronts of what seems to be her lore and part of her gameplay. It was thought that she was going to be a high-mobility ADC, and that was showcased in the trailer with her having a bounty of dashes. Her attacks also seem to be pseudo-ranged (much like Thresh), as she uses a whip rather than projectiles. A part in the trailer also shows her completely negating an instance of crowd-control from Twisted Fate.
Lore-wise however, she seems to reside within Bilgewater or in a region adjacent to it as shown in the trailer. She also appears to carry or pray to the power of a “demon” as she calls it in the trailer, with that being the reason she can harness water abilities. Said demon is speculated to be the “demon of joy” as they call it, also hinted at by Nilah’s overall demeanor and visual design.
Fan feedback regarding the trailer was a bit of a mixed bag. It was hard to say if the comments regarding her mobility were satire or genuine, though, knowing the community, it’s probably a bit of both. League of Legends’s previous champion also had somewhat high mobility, and that seems to be a bit of a pet-peeve with fans as faster champs are a bit more difficult to play against.
Whatever the reaction may be, there’s no doubt people are excited to pick her up. With people pointing to both her cheery personality and fun-looking playstyle as to why they want to try her out. However, Nilah also has the potential to be the next overloaded champ, with people already dubbing her the “water Yasuo” liking the two’s dash-y playstyle. She is speculated to release along with patch 12.13, dropping on July 13.