'A' is an Israeli Jewish minor charged with acting as an accessory, to Amiram Ben-Uliel, in the Duma arson murders.
This article seems to say that he is the same person as this un-named minor from the West Bank settlement of Tzufim, while this article suggests that he is Elisha Odess, whose father is the community rabbi in Tzufim, so it seems as if he actually is Elisha Odess - ptesumably the 'A' comes from the א (aleph) in אלישע (Elisha).
Discussing 'A', this article says:
[Amiram] Ben-Uliel scouted Duma with the minor charged as an accessory, who allegedly participated in the planning of the attack.
This article says:
For nearly two weeks most of the suspects refused to talk to investigators. Then Attorney General Yehuda Weinstein gave the Shin Bet a green light to use torture during the interrogations. ... The use of torture, under the supervision of the courts, broke A., who apparently led investigators to Ben-Uliel. Ben-Uliel eventually confessed and reenacted the crime, presumably under similar pressure.Events involving ‘A’ - Israeli Jewish minor charged as accessory in Duma murders: