And Impersonation - which helps a lot to get into LoS as Triangulated Fire is an Entire Order. (In addition to Surprise Shot) (edit: Of course, Impersonation does not matter when shooting into CC, but in a lot of other situations like vs ODD snipers and stuff like that...)
Plus Keesan Covers a huge chunk of the Classifieds for you, and is a convenient soloist that doubles as filler in broken up triads. I'm a proponent of Keesan now in case no one could tell yet ;)
Countermeasures is really bringing me around on Keesan, she really does it all and while fragile by tohaa standards is pretty fairly priced for everything she is packing.
I'm glad someone mentioned Surdas as strong anti-TAG tech. I'd add that post-FAQ, a fireteam with a couple makauls is absolutely savage as well and will crunch out basically any TAG - but is slower on the advance.
If I take Hatail Keesan I probably take the viral loadout because I personally think that Viral is stronger than K1 in nearly all cases
It's worth noting that "things I want to hit with K1" and "things opponent will leave in B2B with my big stuff to tie it" categories do not overlap too much.
Back when I started Aelis Keesan was our only K1 and we didn't have access to anything that could put a dent in TAGs outside a pack of Makaul's. She would always be K1 and in every list I ran because my buddy loves his Pano TAGs and HI. With the Kosuil and Sukeul K1 is easy to get, so now I tend to stick with Viral Combi on Aelis unless I know someone is packing a multiple TAG list.