Anyone correct me if I'm wrong, but in Japan, they have a pretty popular comedy routine involving duos- one plays the 'straight man' (Tsukkomi) and the other plays the 'funny man' (Boke). The boke makes the jokes and the tsukkomi reacts to them.
A famous Tsukkomi character in anime is Shinpachi from Gintama. If Gintoki is sticking his head in a vending machine slot looking for a time machine, then Shinpachi absolutely cannot ignore it. His job is to point out the obvious, usually in an exaggerated or passionate manner.
As we see in this manga, the FL will often have strong moments of emotional expression, passionate fervor for maintaining the tropes; she reacts to most points of conversation, questions most people's sense and so on. Essentially, she's suppose to be the self aware, grounded one in all the ridiculous.
Anyone know what is the meaning of "Tsukkomi ex" it's say Aria had it as a skill?