Type-Moon held on July 30 and 31 the Fate/Grand Order Fes. 2022 ～7th Anniversary～ event, where several announcements have been made regarding the Nasuverse - In particular, Neco-Arc and Mash Kyrielight will be added to Melty Blood Type Lumina as free DLC characters this Summer.
Mash Kyrielight, also called Shielder, is one of the main characters of mobile game Fate/Grand Order, appearing from the beginning of the story. Seeing FGO is by far the most popular part of the Nasuverse, many fans believed it was only a matter of time till a character from there joined Type Lumina. Type-Moon kept things simple by picking Mash, FGO's poster girl and one of its most popular characters.
Meanwhile, Neco-Arc is a gag, Super Deformed (Chibi) version of Arcueid Brunestud, one of the main characters of Tsukihime. Neco-Arc was playable in the previous Melty Blood game, and reappeared in the PS4, Switch Tsukihime remake, Tsukihime A Piece Of Blue Glass Moon.
It's also interesting to note that when Melty Blood Type Lumina released in September 2021, Neco-Arc became more popular than ever outside Japan after being featured in many Twitter memes. Neco-Arc is currently more popular than Melty Blood/Tsukihime itself, so her addition in Type Lumina could bring in new fans. Personally speaking, I think this is one of the few good things stemming from social media: (younger) people getting to know characters or franchises they would have never encountered otherwise.
You can find the trailers for Mashu and Neco-Arc below. As usual, the characters have specific voiced lines for each other, both in intro and during the battle itself. New BGMs and stages are also featured.
As a side note, keep in mind Melty Blood Type Lumina uses machine translation for its English promotional material, so their tweets are often filled with small mistakes. Most notably misgendering the characters.
Type-Moon also announced two more free characters will be added to Melty Blood Type Lumina in Winter. Lastly, this Summer together with Neco-Arc and Mash, the game will also receive an update adding new scenarios and cutscenes using the characters' sprites. You can see some of these new cutscenes with Noelle in the FGO 7th anniversary stream at the 1:21:54 timestamp.
Last but not least, a collab between PSO2NGS and Melty Blood Type Lumina was also announced, with further details coming at a later date. A new Type-Moon Times stream on August 8 will also include new info on MBTL and Mahoyo. Melty Blood Type Lumina is currently on sale on PlayStation 4, Nintendo Switch, and Xbox One.