Building upon the last volume, we see the relationship progression form a triangle: Takeuchi, Yano, and Nana. Nana has feelings for Yano, but she doesn't seem--when we get to the bottom of it--to be unable to love Takeuchi; the two men love Nana.
We see in this volume a desire to question what is fair in love--not necessarily what is moral, but what is 'fair' or righteous. Takeuchi's sister, for instance, discusses with her brother this very topic, suggesting that 'all is fair in love and war', and that he should find--within himself--a justification for stealing Nana from Yano. (This all occurs after the two re-unite.) Takeuchi, always mellow, doesn't know how to square this idea with himself: he wants Nana happy, wants to promote this happiness, and cannot justify within himself attempting to seduce her; he cannot think it is just.
On the other hand, Yano is completely capable of doing all these things. The so-called noble sentiments of Takeuchi and his sister are nowhere to be found in him: he is a person who wants Nana, whose desire for her is such that the death of his friend is good, that the coercion and kidnapping of him is good, if only he can have Nana. Yano's feelings, throughout the series, are turbulent. His issues about his father, his mother, his dead girlfriend--primarily, his issues of abandonment and trust surrounding women--are precisely the thing which clogs his mind. Is it not strange that he would allow Nana freedom from him, desire her consent, if he is willing to manipulate the scene--even to kidnap and lock his friend up--to be able to seduce her? His actions seem contradictions--his good, his bad--they mesh together to form his personality, such that what is 'noble' about him is hidden, elided, or doesn't exist at all.
The articulation of Yano's trauma--from his mother, from his lovers--indicates a profound distrust of people around him. What he wants, more-or-less, is a love he can accept. He is able to grasp at this love, because he is unable to cope with his abandonment issues; he cannot confront what troubles him, causing him to fly from any dangerous thoughts, from any dangerous ideas, even if they harm his relationships. He is unmanageable precisely because some things become massive issues unnecessarily, because what he wants from others inflates in his mind--precisely because he has no trust in others--and that expansion threatens to burst or to shut down all ability to interact with him. One is interested in knowing just how we come to some conclusion here.