What's happened between...

Since I only have volume 20 to 25, so I'll just tell you what's happen....

Yamanami Suicide
Around volume 10, Yamanami joined Itou-hen. Actually he wants to persuade Kondo to be Joui partial, but then he realized that althought the shogunate already rotten, Kondo wouldn't betray them. He doesn't want to betray any of his friends who has already been together with him since in Shihekan-dojo.
So, he planned to commited suicide by pretend to run away from Shinsengumi. Souji assigned to pursue him. Since Yamanami didn't want to tell anyone his reason for running away, he must do Seppuku, and with his request Souji assisted it.

Akesato, after...
Sei, desperately tried to prevent the seppuku, went to get Akesato. Akesato arrived at time, just to saw Yamanami's death with her own eyes. Shocked and losing hope, she attempted suicide just to be stopped by Ma (a boy who lose his parents and voice after fire incident, he was close to Yamanami and Akesato).
Few days after letter from Yamanami arrived, told her that he had paid enough for her to be free. Sei bought Akesato (Not Geisha anymore, so now Osato) a house, in which then Osato and Ma lived.

Todo Heisuke
He was at those time, not present. He was shocked as he received the news. Hijikata made Heisuke blamed all to him, so that he wouldn't hate Kondo. But surprisingly Ito convinced him not to blame anyone. This shocked Hijikata, as he expected Ito to take advantage of the situation and recruit Heisuke....

Weird new recruit
A new recruit sucpected that Sei is a girl, from the moment he saw her. Of course, none believed him, after all, Sei has already been in Shinsengumi for years. So he attempted to blown Sei disguise, which all failed.
After observed her for some times, he knew that Sei likes Okita and tried to convinced her to marry him instead (which Okita supported, geezzz).

Hogen Matsumoto
New doctor who has studied western medicine, turned up in Shinsengumi. He was a friend of Sei's father. Sei's disguise was blown up from the first meeting. At the end, he helped Sei, by telling the Shinsengumi, that Sei suffered Nyoshinsen, which transform her slowly from man to woman. Therefore, convinced the 'new recruit' that Sei is really a man.

Okita, getting engaged
Some doctor's daughter felt in love with Okita at the first sight, thought the man himself didn't remember, but it was at the fire incident, Okita helped her.
Her father then contacted Shinsengumi, and after few pesuasions from Hijikata, Okita agreed to meet with her.
Sei, sad, dressed up as a girl for the last time in attempt to forget about her-woman-self and came to the meeting place between Okita and the doctor's daughter. Okita saw her from a far and after her.
She got herself trouble with some rufians, since she wore kimono, she cannot fight. But then got helped by Saito and Okita.
Some of her face powder residued on Okita's kimono + Okita tardiness = the ruin of the meeting + the daughter felt in love with Hijikata
Some men saw Okita pursuing a girl with Kimono in Bushi-class, which lead up to Hijikata's suspicion of Okita already has someone he loves.

Photographer
Sei and Okita helped a photographer, who was once a victim of Shinsengumi's fakemember. After the truth was known, as an apology, he wants to take a picture of Okita and Sei for free.
At the studio, seeing many costume and wigs, Okita had an idea to dressed Sei as a girl in the Photo....