Best supporting class for a 3.5 low level campaign?

I’m adapting the FrozenSick campaign from the Explorer’s Guide to Wildermount to version 3.5. It’s going to be the first campaign I play as a DM. The party consists of only 2 players, a warlock and a dragon shaman; the warlock’s player is pretty experienced while the other one is totally new to the game (and to roleplaying in general).

While I don’t want to steal the players’ spotlight, I’m worried that fights could get rough without any other character, so I’m considering adding an NPC who could support them in battle, but I’m having trouble choosing the class.

My addition would primarly be a supporter and/or a utility character (buffer, debuffer, healer…).

As far as manuals are concerned, I’m using PH, PH2 and the Complete Series (all four).

Which class choice could best fit my situation, considering that the characters’ level goes from level 1 to level 3-4 through the campaign?