Sorry about being so late with this, I went on vacation so I didn't have a chance to do it.
Anyway, here's a pretty short/basic summary.
Spoiler:
So, Vol 6 continues where Vol 5 left off, going over Chizuru's past after moving in with her Grandmother. It shows Chizuru's relationship with her and her continued infatuation with Rin that continued even into college. It affects her so much that she can't touch or date men at all. It's shown through the first few parts of this that she often has sexual fantasies of Rin and even includes a hilarious two page fantasy drawn in a slightly shoujo style with a naughty looking Chizuru. In fact, it gets so bad, she ignored the program she was in and moved back to her hometown just to see Rin. It goes over some of the events we've seen in the previous chapters before switching back to the present.
The next major section in Vol 6 is an Aquarium date between Rin and Makoto. It's pretty clear she's trying to get Rin to sleep with her so she can get over the effect of the Stigma but it's not going as well what with Rin being all moody and angsty. They run into the Chizuru's grandmother (which is unknown to the two of them at this point) and Lisa at the aquarium too.
So they're going home and walk by a hotel and Makoto decides to make her move here. She and Rin are at the beach/cliff and she basically goes "I don't care how I do it, I have to! And is almost ready to push Rin into the water so they have to go to the hotel to dry off where she'll get into his pants. She's about to push him in and almost puts her hands on him until he suddenly gets up knocking her back. She almost falls and hits the ground but Rin catches her in an embrace and Makoto just goes crazy and orgasms from it even letting her voice slip. Rin doesn't think of it because Makoto covers it up by saying she may have sprained her ankle. So Rin says he'll give her a piggyback ride back to the parking lot. She refuses at first but the thought of riding on his back gets her imagination going again.
She ends up taking him up on his offer and away they go. Of course, Makoto teases him for having trouble carrying her up when he starts breathing heavily but she's the one that's breathing heavily too. Being so close to him and his hands on her butt has made her insanely aroused and on the verge of orgasming a bunch. Rin then moves his hands from her but to her legs and she realizes that Rin is a little embarassed and she thinks he's recognized her as a girl which causes her heart rate to skyrocket even more and she's worried her heart beat is going to be felt by Rin. She's super embarassed by this (insert really cute two page spread).
They get up to the top of the stairs and run into Lisa again who's looking for the old woman she's with. They agree to help her and Rin runs off to go look for the old woman. Makoto is feeling a little jealous but tries to reassure herself that she only wants his body once to be rid of the Stigma's effects trying to convince herself that the emotions aren't actually real.
Of course, the old woman shows up and is sitting on a bench near Makoto. Makoto talks with her a bit while waiting for Rin and Lisa to come back and comes to the realization that this is Chizuru's grandmother and figures out the entire reason why Chizuru is marrying what's his face is because she could get her grandmother treatment easily at the hospital if she did. Knowing that Rin wouldn't leave Chizuru alone Makoto swears that she won't tell Rin.
Fastforward to the next day at Makoto's parent's house and Makoto wakes up and walks into the living room. She sees her Dad and Rin's lunch on the table, "If I made it, Rin would be happy eh" and then goes through the routine of just saying "Just have sex with him once, not this I'm in love with thing again." She talks to her father a bit then goes downstairs passing Rin on the way, she starts imagining things about Rin and wets her pants thus having to go back to her room to change. She goes by the living room and over hears the conversation between her father and Rin about how folks in the same profession are prefect for each other mentions Makoto as a possible bride. Cue Makoto's fantasies again and her punching the wall hurting her hand.
Switch to Chizuru making dinner for her friend and a slight recap of the situation as a whole. Chizuru realizes her "love" for Rin is messed up and that what's his name helped her a lot by doing everything he could to get Chizuru's transfer done so she could be near Rin and getting her mother into a Group Home with very few people. She feels she has to repay him by "loving" him. Chizuru's all stoic and the like and saying "Rin's said I love you to me. That's more than enough happiness for a lifetime" to herself.
Next section you see Chizuru's husband to be (I'm too lazy to look up his name in the past volumes) sleeping with another woman. A few scenes here and there, and Takako ends up seeing him leave the hotel with said woman.
Takako later meets up with Makoto telling her about how she should hurry up and get it over with by having sex with Rin. Takako would even tell Rin to sleep with her to get it over with but Makoto won't have that. She's the only person Rin believes in because she believes his story. Takako is a little annoyed and tells Makoto to look over. Makoto sees what's his face with said woman. A bunch of thoughts run in Makoto's head but Takako says to her, "Those emotions you're feeling are all an illusion created by that Stigma. I know, I've experienced it. Sleep with him once and those emotions will disappear!" Makoto laments a bit about this at home.
She then offers Rin a ride home after work because his back is still busted. However, she takes Rin to where that guy is and shows him what's going on with him and that woman. He finds out about the guy's escapades and confronts him about Chizuru. Dude has been sleeping around with a lot of women, Lisa included, and Rin is angry about him marrying Chizuru, the woman acts all haughty and she and what's his name are about to leave when Rin says He isn't going to hand Chizuru over. He says Chizuru will choose him and the woman laughs at him. Rin shows her the Stigma and pokes her in the nipple causing her to orgasm ferciously (new attack, Offensive Orgasm!). The woman falls back on the hood of the care and Rin tells the guy that he won't hand Chizuru over to him and that she'll choose Rin over him before leaving. The woman is still sitting on the hood and is a blubbering mess with her legs wet spread out and drool coming out of her mouth while the guy looks at her unsure of what just went on.
Rin gets back into the car and tells Makoto about everything that just happened and they argue because Makoto figures out what he's going to do. Rin spouts some stuff and says that even he uses the Stigma, he'll protect Chizuru and make it real. This sets Makoto off and she gets in the car drives away leaving Rin there. Makoto curses herself about how Takako was right and that she shouldn't have told Rin. She knew Rin was going to pick Chizuru. Makoto stops the car and is crying her eyes out saying that she knows these feelings are an illusion created by the Stigma but implying that the sadness, jealousy, and shock she just felt feels so real to her.
Rin runs off to find out where Chizuru is and runs up to ask that pervy nurse about where she is but apparently, she's not going to tell Rin without getting something first and Vol 6 end with her on the couch legs spread open at Rin.
Sorry about it being messy, I wrote that all at once and didn't bother to edit it.
Anyway, Vol 6 has alot of really nice panels of Makoto and tons of great looks/cute faces for her. Makoto's my favorite character in this Kitazaki series and I love all the time he spent on her.