Lothran and friends
Throughout the beautiful Cyrodiil, Lothran has collected many friends. A lot of them reside at the Mages and Fighters guildhalls but there are plenty more outside of those. Still, loyalty is strong and Lothran always visits the guildhalls for a chat in all the towns/cities whenever he's there. It's also a good opportunity to repair gear and buy ingredients, of course.
Ongar in Bruma is one of Lothran's oldest friends. He helped the Dunmer out when he was in his early days when money was low. Not needing his services anymore, Ongar nowadays mostly provides comedy by stealing food at Olav's and subsequentially getting hacked down by the guards. Sometimes several times an evening

Methredel in the Imperial City remains a good friend too. While having been a competitor entering a certain guild back in the day

there's no bad blood between her and Lothran and they always talk for a bit whenever possible.
I also have to mention
Beautiful Buffy the Brave Bosmeri Bowgirl of course. They've met on several occasions in the past, mostly in Bruma but also on the shores of Anvil. It was when meeting in Bruma once that she gave Lothran his trusted dagger
Snowflake that he still uses during the winter seasons. Buffs wasn't much into Destruction at the time as in other schools, hence why Snowflake isn't too damaging per strike. But coupled with a Weakness to Frost spell it can still do some decent work.
Other friends that come to mind are the Surilie brothers in Skingrad and Jensine, Dynari Amnis, Algot the Northerner and basically all Khajiit in Imperial City. Worthy of mentioning, Lothran often encounters friendly female Khajiit bandits. Not having met a certain individual before he can walk straight up to them and enter conversation. They only have random rumors to talk about but he can also raise their disposition. Pretty neat! This friendliness also goes for the highwaymen. Even though Lothran chooses to give money upon encountering them, they don't actually take any money

Lastly I will mention Aleswell. Ever since Lothran helped them with their issue they are very friendly towards him and he always stays there for a bit when he's on his way to Bruma. Even the mage down in the old fort, being the source of the issue, is an old friend.