Just like real life, kindness in the Persona games gives you a leg up in the world, giving you access to certain relationships you wouldn't be able to improve otherwise. Unlike in real life, people in Persona can tell how kind you are just by looking at you, and some won't give you the time of day unless you're sporting a Mr. Rogers vibe. Kindness is a stat exclusive to Persona 4 and 5, as Persona 3 only had 3 stats: courage, charm and academics.
How to Raise Kindness
Raising the Kindness stat (or Understanding, as it's known in Persona 4) has to do with doing kind actions, taking care of people and choosing the right social links. Reading books that raise kindness, eating certain types of food and taking care of your plants are all ways this social stat can be raised. Kindness allows access to certain Social Links and certain conversation options that are locked off otherwise. While it's certainly not the most important stat, (knowledge can gain you high test scores, which can net you a boost to any Social Link that notices) it's easy to run into trouble if you ignore it entirely.
Kindness in Persona 5
This social stat is a bit harder to raise than the other ones, as you'll need access to your room's plant and some plant food. To do this, as soon as you start up Persona 5 and get a chance, you'll need to clean your room. Once it's clean, the protagonist will notice a neglected plant just sitting there. Buy Bio Nutrients from the Flower Shop in Underground Shibuya (one of the first locations in the game), and the next time you check your plant before bed, you can start feeding it for a much-needed boost in your social standing-- just make sure to keep stocked up on plant food.
You can also work at that flower shop for a boost and some cash- it's called Rafflesia, and you can work there by checking out the poster on the wall near the station.
The second way to raise kindness is to spend time with your caretaker Sojiro whenever you get a chance. He's letting you sleep at the coffee shop despite being a "problem child" due to your run-in with the cops and has a bit of a rough tongue, but as your protagonist gets to know him, his kindness will increase.
If you take care of your plant and hang with Sojiro (who also teaches you to make coffee, which increases Guts) you should be on your way to the first perk--your Confidant Ann, one of the first party members, won't start a relationship with you (friendly or otherwise) until you've brought that stat up. Futaba, a later party member, also needs a rank 4 kindness stat to get past a relationship level of 2.
If you're still struggling to be kind, you can always buy books at the bookstore or DVDs bought from Scarlet, both located in Shibuya. Just be sure to get the ones that increase kindness (the game will tell you which ones those are.) You can read on the train, in your room or the library, and watch the DVDs in your room. If that's still not enough, work at the Crossroads Bar in Shinjuku, once you meet the reporter later in the game. Or order a nostalgic steak while studying in The Diner in Shibuya.
All of these options open up over time, but even if you can't max your kindness in one playthrough, don't worry-- there's always New Game+ to charm the pants off everyone (though mainly Ann and Futaba.)
Kindness in Persona 4
Kindness in Persona 4 is labeled "Understanding," and has five levels, ranging from "Understanding" to "Saintly." Saintly Understanding allows access to The Tower Social Link, a boy named Shu Nakajima who you can tutor in school and in life.
In addition, Understanding helps with the Hanged Man Social Link, Naoki Konishi. Naoki has to deal with being the brother of a murder victim, and your Understanding is key to breaking through his annoyed exterior. Naoki is understandably jaded at having everyone feel pity for him, and having maximum Understanding will allow you to choose "Just let it all out, Naoki" near the end of your friendship, which yields the greatest Social Link boost.
This stat in Persona 4 can be raised by reading certain books on your sofa, eating special ramen on rainy days at Aiya's and working at the day-care after school. Making origami cranes in your off time will also give you a much-needed boost.
Books can be bought at the Yomenaido Bookstore in the Shopping District, while the Daycare Position opens up as a part-time job after May 2nd in the game. This part-time position also opens up a new Social Link- the Temperance Social Link, Eri Minami. Working there is a good way to boost both your Understanding and get closer to this frustrated but caring mother.
The final way to increase Understanding is by making origami cranes- that's a part-time job that increases the social stat and sometimes nets you healing items.
While kindness isn't the best stat to raise, your time in Persona 4 and 5 should be enough to raise it alongside the others. Maxing out your Social Links will grant you access to high-level Personas to use in the dungeons late-game, and these powerhouses can only be fused with the items you'll get if you stick those relationships through to the end. So grit your teeth, water those plants and you'll be well on your way to your goal.