The Giant Blacksmith will consider the player his friend should they talk to him often.
"You good friend. I very happy."
When you first meet Rhea, she speaks very coldly at you, even calling you uncouth. After you rescue her from the Tomb of the Giants, it's really heartwarming to see how much she has warmed up to you. It even seems like you're the only one she trusts by now.
Quelana's gratefulness towards you before she departs, specially this line:
"Our time together was short, but sweet..."
Any time two or more players engage in jolly co-operation to beat a particularly tough boss. Even more so for the one doing the helping, especially if the one they helped gives a bow or a Praise the Sun before the phantom departs.
There can even be cases of this with invading players. For example, being invaded at the Undead Burg and the invader dropping Titanite Shards, which aren't normally available that early.
Dusk of Oolacile's gratefulness to you after you rescue her from the Crystal Golem. And her happiness at being summoned by you after being alone for so long.
"My home of Oolacile was reduced to ashes, long ago in my time. I have been alone ever since. But to be summoned thus, and be of service to thee...it is...most rewarding."
It gets better if you complete the "Artorias of the Abyss" DLC. When you see the state of her time period, it becomes evident that besides Elizabeth, you are her only friend, and a little thing like a few centuries of time apart won't keep you from reaching out to her. And you rescued her again for good measure.
If you donate enough Humanity to the Daughter of Chaos, eventually her voice will lose the in-pain tone. Even if its only a small amount, you've managed to alleviate at least some of the poor girl's suffering.
And after they hollowed, they absolutely refused to attack her, if you take the fog wall as being there just for gameplay reasons.
Learning the truth about Artorias' sacrifice. He never made any deal with the Abyss, ventured into it to save Oolacile from it, and abandoned his shield to protect Sif from its Dark. Then Manus broke him. He was always on humanity's side.
If you rescue Sif as a cub in the past then face him in present, Sif will actually get the drop on you. But he refrains from finishing you off when he recognizes you. Sadly, he can't let you go without a fight without violating his oath, but he at least gives you the chance to defend yourself.
Any time you're invaded or invade someone else, and both combatants give a bow before fighting, often times waiting for the other to finish completely. It's a very simple gesture, but one that can't be interrupted and leaves you open for attack should the other person be feeling a bit cruel. In short, a potentially very stupid move. Regardless, the majority of players do it purely because it's the most sporting option available, and a general indicator that what will follow will be a fair fight.
While a minor example, seeing a message written by another player that simply says "I did it!" after an area where a particularly tough foe or a boss was located can count.