“Sebastian is a rebellious loner who lives in his parents’ basement. He is Maru’s older half-brother, and feels like his sister gets all the attention and adoration, while he is left to rot in the dark. He tends to get deeply absorbed in computer games, comic books, and sci-fi novels, and will sometimes spend great lengths of time pursuing these hobbies alone in his room. He can be a bit unfriendly to people he doesn’t know. Could a charming new farmer cultivate the wasteland of his heart? Who knows?”
Visit Sebastian in his room after 10 AM to activate the cut-scene.
Details
You will find Sebastian working on his computer. After waiting for him to finish, he will mention that he is a programmer, and is working on a project for a client. There is a notification on his computer, and Sebastian explains that it is Sam asking to hang out, but he would rather not go out. Robin enters his room moments later, telling him that Abigail was looking for him and will stop by later. Sebastian asks whether Robin told her that he was working, and Robin says yes, but Abigail decided that she would probably stop by anyway. Robin exits the room, and Sebastian expresses his irritation that nobody takes his job seriously.
Four Hearts
Visit Sebastian at his house to activate the cut-scene.
Details
You will see Sebastian working on a motorcycle in the garage at his house. He will explain that it belongs to him and sometimes he goes for long rides alone far away from Stardew Valley. He suggests the possibility of you one day taking a ride with him, which you can either agree or disagree.
Six Hearts
Visit Sebastian in his room to activate the cut-scene.
Details
Upon entering the Sebastian's room, you are greeted to him and Sam starting up a game of 'Solorian Chronicles: The Game' and Sebastian asks you to join. You then get to play through a "scenario" of which you will be graded on:
-"Warrior. I like a direct approach."
-"Healer. I prefer to help others."
-"Wizard. A sharp mind is the most powerful blade of all."
It doesn't matter which one you chose.
Eight Hearts
Walk to the beach on a rainy day after 1:00 PM to activate the cut-scene.
Details
Sebastian will be standing on the board walk, and upon your arrival, he will be surprised to see you outside in the rain. He tells you about how he is anxious around other people, but he does not feel that way when he is with you. He will then take out an umbrella and notion you to stand under it with him.
Ten Hearts
Visit Sebastian at his home at night (8PM) to activate the cut-scene.
Details
Sebastian will be outside of his house and tell you he was about to go out for a ride on his motorcycle. He brings you to a place he often goes by himself to think about his life while enjoying the view of the City. He tells you his true feelings for you and the cut-scene ends with the two of you embracing with the full moon above.
Thirteen Hearts
Talk to Sebastian anytime(?) upon achieving thirteen hearts.
Details
Talking to Sebastian will give you a Stardrop. Talking to him again will lead to some brief dialogue.
“
“Honey... I wanted to give you that fruit as a symbol of my love.”
— Sebastian
Marriage
Give Sebastian a Mermaid's Pendant to begin your Engagement & Marriage.
Details
After giving Sebastian a Mermaid's Pendant, he is very shocked upon your proposal. He tells you he will set everything up and the Ceremony will be in 3 Days. He wears an Purple Suit, matching his Hair Color.
Sebastian's end of the house will have a Design similar to his Room at his Parents House.
Behavior
Sebastian exhibits 2 notable behaviors once married.
1: Taking care of tasks for you
Watering all crops on your farm (only in spring, summer, and fall)
Repairing broken fences.
Feeding all Animals
Watering your Dog's/Cat's bowl
2: Cooking breakfast, adding one random breakfast food to your inventory. If inventory is full, you will not receive a breakfast item. (The Item for each Food does not have to be in the Fridge.)
"Standing at this spot really makes me want a smoke... but I'm determined to quit now."
Dialogue Updates after Marriage
Asking player how their day was.
Telling player not to over exhaust self on farm chores.
Cheering player on during farm chores.
“
“The Stardew Valley Fair is a little crowded for my tastes... but I'll just hang out with Sam. It'll be good to catch up with him. You should play some carnival games,though! Don't worry about me.
Living here with you is teaching me to come out of my shell a little bit. I think it's good for me.
I spent the morning repairing a few of the fences. They should be as good as new.”
Timeline
Sebastian's look evolved over the years the game was in development.
Here's a timeline showing how ConcernedApe's art and Sebastian's style changed over the years before the game was launched.
Magma Geode
Quartz
Gem Node 
Amethyst Node
Cauliflower (1)
Cheese (1)
Wild Plum (2)
Wheat Flour (1)
Sugar (1)
Fish (1)
Seaweed (1)
Rice (1)
Tomato (1)
potato (1)
Oil (1)
Hops (1)
Wheat (1)
Goat Milk (1)
Energy (100)
Health (40)
Foraging (+2)
Farming (+1)
Speed (+1)
