“Who?” The broker your Covenant and Bolvar both send you to. She is the linchpin between Torghast and the Maw. She not only gives you the quests to send you into both places but also sells a variety of items to help you there. Some of the items benefit you only in Torghast, like “first death doesn’t count.” Those are unlocks which are account wide, you only need to buy them once (yay). Some of them are items with charges that are, of course, only for the person holding the item (e.g. Sticky-finger skeletal hand which unlocks things in Torghast). Some of them are buffs for the Maw (like the transportation powers, e.g. Encased Riftwalker Essence or Cypher of Relocation) which have to be bought for each character. Each character has to build up their own reputation with Ve’nari and unlock the additional areas on their own.
All of the Torghast permanent upgrades are worthwhile, especially since you only have to buy them once. I don’t find the Torghast consumables useful. If you are only going into the Maw to get souls, you don’t need the Maw upgrades, otherwise the transport ones are quite helpful. If you are a crafter, she sells the Crafters Mark II recipe (at cordial), which is needed to be able to make higher level items. Some upgrades require higher rep with Ve’nari, but you can complete Torghast without any of the upgrades.
The reason some people push to the maximum rep with Ve’nari is that she will then sell you an item to add a gem socket to any piece of gear (at 7200 stygia a pop). That takes a lot of grinding.
When fighting things in the Maw, you will sometimes get items that give a quest to turn them into Ve’nari for more rep. Ve’nari has a different reputation scale from most factions and is similar to the Fishing Friends from previous expansions. It’s a bit quicker to get to Cordial than it is to get to Revered for most factions.
Ve’nari is voiced by Aysha Selim, who also did Ana Amari in Overwatch.