First, we'll discuss what exactly the pink SOUL does, then we'll move onto attacks and symbolism in-game and all that great stuff.

You're already familiar with what the other SOULs look like and how their different SOUL modes affect gameplay. However, the Pink Human AU is more concerned with the game's lore and in-universe implications that just gameplay mechanics. For instance, during gameplay, the green SOUL mode includes a shield to block attacks. Undyne's dialogue clarifies that she gave the protagonist a spear to "block the bullets with". This means that having a green SOUL in-universe does not automatically generate a shield for the player, only that they cannot escape a battle (which it the reason Undyne makes the player's SOUL green in the first place). The Pink SOUL, however, does generate a shield. A good visual comes from art by kasempan on Deviantart:

In-game battles with a pink SOUL would have a small shield that can be rotated in whichever direction the player wants it to face, all while being able to freely move around the box. The shield deflects normal attacks, but it also deflects green attacks (it doesn't affect orange and blue attacks). Some battles where touching a green attack is necessary to meet SPARE criteria cannot be completed without some risk: either the player must move their shield out of the way to collect the attack, or they must let their shield fade in order to complete the SPARE (such as in the case of enemies like Love Crab and Cepapod, see this post)

So how does one lose their pink shield? By losing PP of course! PP (while being a hilariously immature joke on my part since it's funny to say) is a common video game trope usually used to give player's magical abilities (similar to "mana"). However, in the Pink Human AU, it is used only during a pink SOUL mode to denotate "Pink Points"/"Protection Points". A pink bar is seen at the bottom of the battle box. It begins at 100% each time a FIGHT is started. The shield is at maximum power. The more pink attacks the player is hit by, the more the shield loses PP until it reaches 0%, in which case it disappears completely. PP cannot be replenished in-battle. Once the shield is gone, it's gone. However, it will return to 100% in the next battle, regardless of HP.

I've already posted about the Pink SOUL's "trait", but I'll elaborate in case anyone reading this is unfamiliar with the lore of the Pink Human AU. Each SOUL has a trait associated with it: yellow, justice; orange, bravery; purple, perseverance; etc. The pink SOUL is associated with love (the true kind of love, not LV/LOVE). The pink SOUL having a shield represents a "guarded" heart. This is why the shield also reflects green attacks, which are associated with healing and kindness. A guarded heart is so busy defending itself that it cannot accept good intentions. In order to progress, you must open yourself up to the risk of being hurt.

Monsters express themselves through magic. The book in the librarby makes it clear that bullet patterns to other monsters are akin to greetings (for instance, the "bullet pattern birthday card"). Magical attacks are themed to a monster because they represent that monster as an expression of themself. To monsters, pink attacks represent affection. To a human, pink attacks do nothing. To a pink SOUL, pink attacks break through the SOUL's barrier, opening them up to getting hurt by regular attacks, yes, but also opening them up to being treated with kindness with green attacks.