The KD stands for the initials of K' (Kay-Dash).
KD-0079 is the first in line of the KD series of songs. It mirrors the adventures of K' and Maxima into finding the NESTS cartel and Kyo.
This song is, as its name implies, a nod to K' being the 79th in line of the Kyo clone project.
The song returns in The King of Fighters 2002 albeit a bit faster.
It was remixed for The King of Fighters 2002: Unlimited Match titled "KD-0079+" It features a new intro and has a new hard rock-techno feel to it.
The song is rearranged in The King of Fighters XIV for K' vs. Ralf Fateful Battles.
KD-0084 is the second in line of the KD series of songs. It mirrors the further adventures of K' and Maxima into finding the man behind Project '00.
The song returns in The King of Fighters 2002 albeit with additional notes before the refrain without a drum solo in between.
KD is the third in line of the KD series of songs. It is essentially a remix and remaster of KD-0079, as evidenced by the intro.
This song is noticeably darker than the other KD series, as it mirrors Mukai's quest to unseal the Orochi with K' and Maxima having to stop such a thing from happening.
It also reflects K' past, hence the title being "KD". The electric guitar from the start is a nod towards K' being a lazy teenager living a peaceful life with his sibling. The theme takes on a darker tone which hints towards an evil force (NESTS) is approaching, the electric guitar takes a more depressing tone which signifies K' being held captive and exposed to Kyo's DNA. The theme then takes on a more positive, powerful, rebellious tone which is a reference to K' fighting back against NESTS and soon the new foe Mukai.
KDD-0075 is the fourth in line of the KD series of songs. The song code is renamed to "KDD" to connote the additional "KD" that is Kula Diamond. The theme represents Maxima, Kula and K' in that specific order.
The song is noticeably higher in quality than the other songs of the KD series due to the Atomiswave's hardware, though an arranged version is also possible.
This song is one of the two available themes for the K' Team in The King of Fighters XIII, the other being KDD-0075. The song features electric guitars and seems faster paced compared to other songs in the KD series. Like KDD-0075, the theme represents Maxima, Kula and K' in that specific order. Because of newer hardware that allows the game to play sounds at a better quality, the theme does not have an arranged version.
While not used as the theme for the K' Team itself, this theme is used for the Derelict Church stage in The King of Fighters XIV. It is faster paced, almost as fast as KDD-0063, but features more electro synth instruments. The theme is played like a gospel, in keeping with their stage theme being a derelict church.