OK , so I've just beat the Dragonslayer Armour and had left with 9 etsus flusks and 0 FP.
I reach the next bonfire which is pretty close , I light the bonfire but I don't rest at it , because there is a summon sign.
I summon this "Myla the Thief" which I though at first that it was an NPC but a few seconds he/she jumps around near the elevator contramption which is near the bonfire and I realize that thief is spelt wrong and that he/she is a player.
He/she collapse and then starts to go ahead in the Grand Archives we fight with the Crystal Sage and the other enemies which are new to me , even the hands on the bookshelfs were weird because I didn't use the Wax.
While we fight and advance I start to notice that "Myla" was using only a small dagger and he/she was showing me to use the hidden ladders and levers , he/she even showed me the part where are the gargoyles outsides.
We get outside of the library and he/she collapse , I though that it was because of the statue and that it was something about the story , a few seconds later I realize that we are attacked by 2 NPCs who look human , 1 second later I noticed the archer too .
We win the fight against this trio and proceed further to the bridge with the stairs where I was close to die a few times , yet we reach the end of the stairs and there is a room with a fog wall.I saw a summon sign so I summoned it , it was "Sirris of the Sunless Realms" a true NPC .
I go through the fog and soon they come too, we fight with this boss "Lorian,Elder Prince" where I was trying to learn his mechanics , finally we won .
Or at least that's what I though , the "we won" wasn't actually a win because the boss wasn't dead , it was only the first phase of it .
We looked doomed ,"Sirris" was almost dead and I had only half of hp , 1 etsus flusk and 1 divine blessing. But we fight and fight , I get knocked a few times by the mechanics which I didn't knew. Yet "Myla" was fighting with his/her dagger a boss whom he know that it was my first time. We tried to aggro the boss , one by one , he was hitting him a few times , getting his aggro and then I was doing the same trying to do my best.Even if it looked over I fought with only 20% hp while the boss had half on the second phase.
In the end our struggles pay off.The boss is dead , we rejoice with our gestures and spamming the animation for the empty etsus flusks.
While I light the bonfire I realize that I had 261320 souls and later I realize that I've just beat the last Lord of Cinder "Lothric, Younger Prince" whom he was in phase two of the boss.
And this my friend is the story of my CO-OP journery in Dark Souls 3 with a "Thief" whom had only a dagger, a journey that I won't forget so soon.

Edit:
Fun fact :I had my Estus Flask at +0 ,because I didn't knew that you could make + (Later I've found out through some wiki)