Ethos names the persuasive appeal of one's character, especially how this character is established by means of the speech or discourse. Aristotle claimed that one needs to appear both knowledgeable about one's subject and benevolent. Cicero said that in classical oratory the initial portion of a speech (its exordium or introduction) was the place to establish one's credibility with the audience.
In Cicero's speech defending the poet Archias, he begins his speech by referring to his own expertise in oratory, for which he was famous in Rome. While lacking modesty, this tactic still established his ethos because the audience was forced to acknowledge that Cicero's public service gave him a certain right to speak, and his success in oratory gave him special authority to speak about another author. In effect, his entire speech is an attempt to increase the respectability of the ethos of literature, largely accomplished by tying it to Cicero's own, already established, public character.
The topics of invention serve to increase the ethos or credibility of the speaker insofar as these are logically persuasive, and insofar as one appeals to the kinds of proofs or authorities that are seen as authoritative to one's audience.
Arist.1.2.4, 2.1, 2.12-17