Adder is the arrogant, overconfident, second-in command CO of Black Hole. With no Day to Day powers, and having a very similar set of abilities to Koal, he may seem underwhelming. However, Adder give the player the tools to be able to take games off of higher-tier CO's against equally skilled players, arguably more so than other low-tier CO's.
Hits: His Face
Miss: Dirty Things
Adder is, in many ways, unremarkable. And in Advance Wars, being unremarkable means being Low Tier. As one would expect with a CO with a 2-star CO Power, you spam that in most cases. This means he gets 110/110 every other turn, and nearly every turn on larger maps, which is something to note.
Adder's CO Power is what makes him, and the player should take extreme advantage of it. To make the most of it, the player's moves should be made while taking into account the extra movement range. The Power also comes in handy with re-positioning, allowing infantry to reach their capture target sooner and units to move from bases to fronts. Future engagements can be set up with the bonus movement, often messing with the game of "footsies" that are made by stalemates.
A wary opponent will keep in mind Adder's bonus movement as well, and take the necessary precautions. This, however, does not take away the advantage given by the extra movement. Often times, the opposing player will have to cede terrain or important positions in order to not take unfavorable trades.
While he doesn't have the Road-fueled firepower bonuses of Koal, Adder does have his own edge over him. Adder's CO Power costs two stars, as opposed to Koal's three stars, and that often makes a world of difference. So even when playing on a map with a fair amount of Roads, there is still merit in choosing Adder over Koal.