Referencing Scarlet Weather Rhapsody
Without SWR you will be unable to access anyone except Reimu, Marisa, Alice, Patchouli, Sanae, Cirno, Meiling, Utsuho and Suwako. To get Tenshi, Iku, Suika, Reisen, Remilia, Sakuya, Yuyuko, Youmu, Yukari, Komachi, and Aya you first need SWR installed on it's own. Open up configex123 in your th123 directory and look for a line saying th105path. Change the path you see to your SWR directory and then execute th123.exe. Go to Vs Com to confirm that the rest of the cast have indeed been unlocked.
This only unlocks the CHARACTERS, the decks and other unlocks will not carry over, but you don't need to do anything to get the characters themselves, just reference SWR properly.
However, by placing both games in the same folder, upon first running th123, cards from SWR will be imported. If you have already begun playing, you can delete your score123.dat file, which will erase all cards you have unlocked in th123.
If you do not have Alice and Patchy after adding the SWR crew, beating Cirno and Meiling's story mode on any difficulty will fix that.
Unlock Suwako and Utsuho
Fight them in Story mode or in Arcade mode.
The enormous monster seen in the title of the game is named Hisou Tensoku. This Kappa made action figure is the cause of the incidents in Sanae and Cirno's routes.
Reimu's Fatal KO
For a Hokuto No Ken style finisher with Reimu, activate Fantasy Heaven on a 3rd round successfully, her theme will play remixed to a Hokuto No Ken style. Much like this: http://www.youtube.com/watch?v=8hSWYyi5BjE Reimu's style : http://www.youtube.com/watch?v=O8-SrmzsrkE
Wriggle Nightbug and Nitori Kawashiro were originally slated to appear in the game, but were scrapped due to time restraints. Kaguya Houraisan and Fujiwara no Mokou were also considered, but it was decided that they were too strong. Instead, Tasofro decided to put in mostly weak characters, as they would humorously face off against the very powerful characters featured in IaMP (most every mainstream character was featured there). Sanae Kochiya was added instead of these possibilities, and perhaps made as weak as she is on purpose.