Time LineTales of Symphonia does not place emphasis on exact dates, so this isn't much of a "time line" in that sense. Basically, I've broken down the events in Kratos' life in bullet point form with relative dating (pre-Symphonia, Symphonia, post-Symphonia). Information has been gathered from Kratos' path in Tales of Symphonia, Tales of Symphonia: Knight of Ratatosk and Kratos' story in Tales of Fandom Volume 2.
PRE-SYMPHONIA TIME (~4028-4000 years before)
• Born in the capital of the Tethe'alla Kingdom; Battles of "The Ancient War" were an everyday occurrence.
• Becomes captain of the knighthood of Tethe'alla; Has feelings for his Queen, Soleille.
• Meets Mithos while he is trying to warn Tethe'alla that Sylvarant is trying to launch an invasion. Kratos happens to pass by and wonders what all the commotion is.
• Agrees to help Mithos and Martel, and they try to stop the conflict from breaking out, but the King would not listen to the words of half-elves and the capital goes up in flames.
• Decides to accompany Mithos and Martel on their journey in creating a place where half-elves can live in peace, as he has failed as his duty as a knight. His status as "Kratos of Tethe'alla" may help them gain the ears of people they would not otherwise have. He gives up his privileges as a nobleman and his security as a human and is chased out of his country.
• Yuan joins him, Mithos and Martel on their quest. They decide to look for Fandalia flowers in order to cure the Ozette Flu.
• Martel falls ill with the Ozette Flu and eventually dies. Mithos becomes enraged and blinded with depression because of his sister's death and decides to create a world in which everyone is exactly the same (he misunderstands Martel's wish).
• Mithos makes pacts with all of the Summon Spirits, and turns Kratos into a living seal for his pact with Origin in order to prevent anyone from obtaining Origin's power.
• Kratos drinks Aionis, a magic rock, in powdered form in order to undergo a special "magic transfusion" that uses the body's mana to alter its metabolism and stop the body's internal clock. This allows him to stop aging, use magic and live longer than elves. He becomes a Seraph.
• The order of Cruxis is established. Kratos and Yuan become the first angels with Mithos, as neither of them could stop Mithos' rampage at the time.
• Meets a human test subject in the Angelus Project named Anna. By getting close to her, he realized that he was turning a blind eye to what Mithos was doing and decides that he can no longer go along with his plans. He begins to search for a way for a human to wield the Eternal Sword in order to reunite the worlds.
• He and Anna are pursued by Cruxis and traveled from place to place, and eventually Lloyd was born and they traveled with him as well.
• (~15 years before Symphonia) He, Anna and Lloyd are eventually caught by Kvar, who removed Anna's Exsphere. Since she did not have a Key Crest, the mana in her body went out of control and she turned into a monster. She tried to kill Lloyd, but Noishe saved him and she regained consciousness for a few seconds. Anna begs Kratos to kill her, and when she turns on Lloyd once again, he has no choice but to do so. Kvar attacks and Lloyd and Noishe fall down the cliff along with Anna's Exsphere. Kratos descends the cliff but sees no sign of Lloyd or Noishe, and believes that there is no way he could still be alive.
• Returns to Cruxis because life lost its meaning to him and since he was Origin's seal, Mithos would not leave him alone.
• Assigned the duty of making sure the Chosen of Sylvarant completes her duty of world regeneration. Volunteers to be a guardian of Colette as a mercenary, and is accepted due to the circumstances created by Desians.
• Meets Lloyd, and realizes the grave outside his house belongs to Anna. He can hardly believe his son is still alive.
• Travels with the Chosen's group until they reach the Tower of Mana, and then reveals himself to be one of the four Seraphim of Cruxis. He battles the party when they challenge him to save Colette, but they are then interrupted by Mithos.
• The Great Seed goes out of control after Sheena makes pacts with all of the Summon Spirits minus Origin and Kratos temporarily rejoins Lloyd's group in order to help them restore peace to both Sylvarant and Tethe'alla.
• Yuan arranges a meeting between Lloyd and Kratos at Altessa's house, hoping that Kratos will release Origin's seal if his son is in danger. He informs Lloyd that Kratos is his father. Lloyd attempts to fight Yuan, but he fails and Mithos arrives. Kratos shields Lloyd from a fatal blow from Mithos and is knocked unconscious. He is taken away by Mithos upon his leave.
• Mithos imprisons Kratos in Derris-Kharlan, and Kate comes to rescue him. Once they make it back to their world, he informs Yuan that he plans on fighting Lloyd in order to release Origin's seal.
• Talks to Lloyd in Flanoir, and tells him about Anna and the day that he had to kill her.
• Fights Lloyd one-on-one in the forest behind Heimdall and loses to him. He breaks Origin's seal by releasing all of his body's mana, but Yuan comes at the last minute and transfers some of his own mana into Kratos' body. Lloyd yells at him for wanting to die, and Kratos realizes that if he wants to atone for anything he's done, he has to live.
• Gives Lloyd a sword while Dirk gives him another. Asks to accompany Lloyd so they can defeat Mithos together. Saves all of Lloyd's friends from the impossible situations they put themselves in.
• Joins Lloyd in the battle against Mithos and they are victorious.
• Decides to return to Derris-Kharlan in order to dispose of Cruxis' supply of EXspheres into space. Uses the remains of the Tower of Salvation to return there, saying his final goodbye to Lloyd.
POST-SYMPHONIA TIME (~2 years later):
• Remains with Derris-Kharlan as it travels into the depths of space, drifting further and further away from Sylvarant/Tethe'alla. Keeps in contact with Yuan where the Tower of Salvation used to be, updating him with how things are there, as well as hearing Yuan's side of things. They are looking for a place to live in peace, and eventually become too far apart to make any further contact with Sylvarant. Says his final goodbye to Yuan, and asks Emil to take care of Lloyd and his friends.