Thersites is a railing malcontent who relentlessly offers lacerating commentary, a la satiric chorus, on the foolishness of all. He amuses himself with puns on boils and skin diseases. "Paradoxically, it is the most scurrilous figure in the play, the most nearly sewer-mouthed character he ever created, Thersites, who seems at times to be the author's mouthpiece, acting as a sort of chorus and commentator on the action and the other dramatic persons" (Goddard, II 3). He's a nasty character, but as one who will divide his time "between a vicious thug and a stupid one," he is the only character who has perhaps "an outraged sense of intrinsic value" (Bloom 332). Ajax and Thersites insult each other viciously, with Ajax beating on the latter.
Thersites says that "a great deal of [Achilles'] wit, too, lies in [his] sinews" (II.i.98-99), and he calls Patroclus "Achilles' brach" [Achilles' bitch]. "This is one of the few explicit and contemptuous references to homosexuality to be found in Shakespeare" (Asimov 101). Patroclus thinks Thersites' leaving "A good riddance" (II.i.120) -- the first appearance of this famous phrase, despite the OED listing 1782 as the first occasion. Achilles and Patroclus may, in the latter 1599 revision of the play, represent Essex and Southampton (Anderson 315-316). Bloom leans towards the notion that Achilles is Essex (327).
The elder Ogburns detect an early version of the play in which Thersites, a kind of "puritan-agitator" (see II.iii.21), does, like Caliban, function as the voice of the puritans (Ogburn and Ogburn 618). In a late revision, carried out during literary wars, Thersites becomes partly Ben Jonson, partly Chapman (Ogburn and Ogburn 601, 951) whom Oxford is retaliating at with their "classical weapons" (Ogburn and Ogburn 953).
A letter from the Greeks to King Priam indicates that the war would end if only the Trojans return Helen. Hector is for it, Troilus and Paris against. Although Troilus considers "brother priest" Helenus' pacifism a matter of "dreams and slumbers" (II.ii.37), he rejects "reason" himself, openly. An analogy about "remainder viands" [left-over meat] (II.ii.70) attempts to help the case, but "Helen would hardly have been flattered by such an argument for her retention" (Goddard, II 24). "Troilus believes in subjectivity -- which, as we know, will be his downfall" (Wells 221).
Madness bursts out in the middle of the scene in the person of Cassandra, shrieking about Trojan doom. "Shakespeare's state of mind when he wrote Troilus and Cressida ... must have been something like Cassandra's" (qtd. in Goddard, II 4).
The debate resumes and Hector admirably states the logical and lawful case: that Helen should be returned but then becomes suddenly and totally inconsistent in recommending that they fight on (II.ii.163-193). This is a 31-line speech, like the prologue. He seems to shift finally due to the allure of "dignities" -- reputations.
Troilus is elated and "with characteristic romantic logic proves the goodness of their cause by the fact that they are fighting for it!" (Goddard, II 25). Hector's switch has taken the matter "From truth to -- dignity. From wisdom to -- fame. From heroism to -- glory. But most of all from one's own soul to -- what everybody thinks and does" (Goddard, II 27). Troilus speaks in terms of necessary excess: some things are priceless" (Garber 556).
Thersites goes over to Achilles' camp, as Achilles considers him "a privileg'd man" (II.iii.57), welcome to rail so long as he remains entertaining. The other Greeks make appeals to Achilles to no avail, Ajax calling him "lion-sick, sick of proud heart" (II.iii.86) -- although there is every reason to suspect Achilles may be sick of all the other Greeks' pride. Ulysses, Nestor, Agamemnon, and Diomedes all privately mock Ajax.
Detestation for proud men (II.iii.170) refers to Achilles, "the allusion being to Leicester whose pride was infinite" (Clark 632).