Just read the trans for 410.
Spoiler for 410:
The new girl is named Rimi and she tends to speak in the third person. The reason Chikage joined the group in their little adventure is because they trick her in believing that they are solving a mystery.
She is in child mode under the pretense of being a part of the young detective squard. It appears they are rescuing the Detecive that helped out in the past arc and that he participates in the fight. The reason the police are after the masters is because the ones that are Pro-YAMI have outnumbered the ones that were Anti-YAMI.