Two possible answers.
1) if Gal 1:19 was found in the original text, then James is a negative figure in the eyes of the original writer (because, assuming he was the Pillar of Gal 2, too, he is the leader of circumcision party that is hostile a priori against Paul).
2) else (if Gal 1:19 was not in the original) James is a positive figure. In that case his introduction in Gal 1:19 was made by a Catholic interpolator to make an anti-marcionite point: ''Paul'' observes the 'fact' that Jesus had a biological brother. The interpolator therefore was inspired by Hegesippus, the first Catholic author to mention a biological brother of Jesus, called 'The Just' against Marcion calling the Demiurg as 'The Just'.
But under the hypothesis 2, I find a problem:
Detering opts for the original marcionite version:For before certain men came from James, he used to eat with the Gentiles. But when they arrived, he began to draw back and separate himself from the Gentiles because he was afraid of those who belonged to the circumcision group.
The Catholic interpolator didn't like denigrate in first place Peter, therefore he did prefer replace him with ''some from James'', a minor figure in a Catholic universe.For before Peter came, he used to eat with the Gentiles. But when he arrived, he began to draw back and separate himself from the Gentiles because he was afraid of those who belonged to the circumcision group.
But I think, against Detering, that for the catholic Hegesippus James wasn't just a ''minor figure''.
I think that for early proto-catholics, James was their anti-marcionite icon (finding his basic raison d'etre in his being used against Marcion in Gal 1:19). Catholics did cast James in a cousin of Jesus only later, after the II CE (after the heretics did coopt the figure of James, too).
But in this case, how do you explain the view of James in Gal 2:12 as a positive Catholic figure in the eyes of a Catholic ''Paul''?
At moment I should still meditate about, but I think that an original Catholic exegesis of Gal 2:12 (in his actual state now) may be this:
James has the role of Observer and Watcher with simple anti-heresy function. He is not a Judaizer but only the James of Acts (later converted in a brother of the Lord by Hegesippus and by Gal 1:19). That James decides that the Gentiles should be only respectful of the 7 noahide laws and only these: therefore no Torah for the gentiles. Therefore the ''some came from James'' are not real ultra-Judaizers and therefore not enemies of Paul. They weren't pressing on Peter in no way. But Peter was the guy that, because his excessive Judaizing zeal and/or likely his strong fear of the ''false believers (that) had infiltrated our ranks to spy on the freedom we have in Christ Jesus'' (Gal 2:4), betrayed Paul with his defection from the gentile table. The sin of Peter was to fear the 'some came from James' by exchanging them mistakenly for true Judaizers when they weren't really these people, but were only normal observers send by James the Just. In this way the Catholic could accept that error of Peter without for that having to assume an a priori hostility between James/''some from James'' and Paul.