https://jp.reuters.com/article/jal-lcc-idJPKCN1IF0CB
A bit more info from Reuters.
1. They want to make it profitable within 3 years
2. Prefer to use a separate brand and you probably won't see the Tsurumaru logo either
3. The 787-8 will have ~300 seats, a 40%-50% form the existing ~200 seat config on JAL