If you have 450 or more Relationship EXP with them you will get their "best" ending.
If you have 225 Relationship EXP or less with them, you will get 1 Heart and the "okay" ending.
If you have more than 225 but less than 450 Relationship EXP with them, you will get 2 Hearts and the "good" ending.
Gifts are a great way to increase your Relationship EXP.
You can't give gifts to characters right away after meeting them.
But how are you going to give it back to me if you're leaving? You will be able to meet Emmett again at The Train Station on or after Day 10.
Using the cheat code donottrytrainjumpingathome can also be used to unlock Emmett.
Talking to characters will increase your Relationship EXP with them.
Once you have enough successful dialogues with the character, a message letting you know that you can now give gifts to the character will appear.
You will remain in the era you are currently in so travel to the present if you are in the past before the last day if you would like to end your game in the present and vice versa.