The Mask ending of the 2.0 update not only requires you needing to recruit the shadow, but also complete their fourth encounter before day 6 (10 Days Remaining). This puts you on some pretty strict timelines depending on other things you may want to be doing on your run. Its also extremely easy to mess up the Shadow's affection potentially losing you hours of progress.

To recruit the Shadow we need to encounter them several times throughout the apartment while being kind to them. If their affection is high enough (8 or more) they will give us a rose in their 4th encounter which guarantees they will appear in our bedroom. The rose is also required for the Mask ending, since we need to give it to Charan in the basement by day 6 (10 Days Remaining) at the latest. The Shadow has a chance to spawn in any room you have already entered starting on Day 2 (14 Days Remaining). In general, it will remain in that room until interacted with. According to the miraheze wiki you can get the 1st and 2nd encounter on the same day, but I've only ever gotten them on separate days and will be using that as the assumed timeline.

When first encountering the shadow we want to select "Hello...?", "You seem okay...", and "Are you a friend?" These will put us at 2 affection. Next ask "Do you need help?" and "Are you hungry?". The muffin from Jeane's apartment will give +3 affection and give more flexibility in encounter 3. Giving a chocky bar or smaller food will force you to give a gift in encounter 3. If you do not have anything you can or want to offer them a the moment, you can select Nothing and then "Sorry, can't help now." for no penalty. They will remain in the same area until you give them food.

After giving food we will have at least 3 affection on the end of day 2 (14 Days Remaining)

I've never managed to get this on the same day as encounter 1, so we're going to assume its now day 3 (13 Days Remaining). When encountering the shadow select "Hi again, friend.". Then select any or all of the questions in the following prompt. As long as you ask 1 question you will go up 1 affection. Be careful when menuing as selecting "Attack" will give you -3 affection even if the Shadow has no reaction to it. This is how I messed up my save and started looking into this.

We now have a minimum affection of 4 at the end of day 3 (13 Days Remaining)

Day 4 (12 Days Remaining), find the shadow and they will ambush you to give you a gift. Take the gift, and the next choice will depend on the food quality you gave in the first encounter. If you gave them the Muffin or a higher quality item you can simply thank them for +2 affection to get to your 8 affection. If you gave a lower quality item you must give them a gift in return. This can be a wrapped gift, vendor trash, or key item that no longer has a use (Ben's army man for example). This will give you +4 affection. Unlike other days, they will not remaing in the room until you interact with them. If you leave the room they will despawn, but they can keep respawning in eligible rooms until you receive the gift from them.

So going though the math of the previous interactions we have
+2 Encounter 1 Dialogue
+3 Muffin
+1 Encounter 2 Dialogue
+2 Thanking them for a gift
Giving us a total of 8 affection

Alternatively we have
+2 Encounter 1 Dialogue
+1 Hard Candy
+1 Encounter 2 Dialogue
+4 Giving a Gift
Also giving a total of 8 affection

We should now have all the required affection to get the rose the following day.

Day 5 (11 Days Remaining), when leaving the apartment the shadow will be outside and they will leave you a gift after talking with them. If everything was done correctly this will be the rose which you can now take to Charan today or tomorrow to progress the Mask ending assuming you've been working on getting to the basement this entire time. If you receive any other reward, it means you do not have enough affection. The lower rewards are as follows

6-7 $350
4-5 3 x Tonic
2-3 Tonic
1 or less Kitchen Knife

At the start of Day 6 (10 Days Remaining) Shadow will be in your room. Resist the urge to walk into the cloak, then ask them to behave and they will join the apartment. They will give you a free item each day, and you now have access to the Mask ending assuming you gave the rose to Charan in time.

Assuming you've gone through enough of the events to get encounter 4, and have a save file before it, you can just edit the save to change the affection. If loading into https://www.saveeditonline.com/ its variable #152 as of build 2.17. Just set this to 8 and you will get the rose and encounter 5 the next day. If you missed getting the rose to Charan, can't help you there. Charan leaves on Day 8 (8 Days Remaining), so the rose needs to be given by day 6 (10 Days Remaining) to access the pit on day 7 (9 Days Remaining)

Shout out to the wiki for giving some concrete numbers on some of these events. Made trying to compare saves on where I messed up my run much more tolerable, and helped finding the save variable to fix a lot easier.
https://lookoutside.miraheze.org/