Most likely they will not be added considering how easy is it to obtain BB in HeRO and the reward that comes with those MvP. (Gloom= free OCA, Kiel= 2nd best exp boss monster for example)
Cirno: ⑨⑨/70 BAKA BAKA ⑨ High Wizard
Reimu Hakurei: 99/70 High Priest (Purge Evil Spirit O_O)
Sakuya lzayoi 8x/5x Sinx
And even if they do spawn the mini-boss of Nidhogg, you have to be at least level 101 to spawn Rata. (Aqua Elemental- Level 103, Duneyrr- Level 109, Nidhoggur's Shadow- Level 145)
Cirno: ⑨⑨/70 BAKA BAKA ⑨ High Wizard
Reimu Hakurei: 99/70 High Priest (Purge Evil Spirit O_O)
Sakuya lzayoi 8x/5x Sinx