When Lexa the commander of the 12 clans met Clarke they were enemies before Clarke made an alliance but at the expense of Finn her first love to compensate for the death of all the grounders killed, but Lexa comforted Clarke telling her about her first lost love Costia. Lexa and Clarke worked well together and eventually Lexa confessed her feelings but Clarke said she wasn’t ready for a relationship. Lexa eventually betrayed Clarke by abandoning Clarke and her people in exchange that their enemy free her people. The alliance successfully took down Mount Weather's defenses, but before Lexa's army could attack, Cage Wallace offered Lexa a deal that ensured all of Lexa's people would be freed. In return, Lexa's army would have to retreat and never encroach upon the Mountain again. Lexa took the deal, saving her people but leaving Clarke and the rest of the Sky People to die in Mount Weather. After Clarke was forced to irradiate Mount Weather, the deaths of all the Mountain Men and Clarke's post-traumatic stress causes her to hate Lexa for her betrayal and for endangering Clarke's people.

Months after her betrayal, Lexa found herself struggling to maintain control over the Coalition; her rule being challenged first by Queen Nia of the Ice Nation, and then by Pike's faction of the Sky People. She also had Clarke brought to her in Polis amid the political turmoil. As Lexa worked to bring peace to the clans. Lexa worked to earn Clarke’s forgiveness though Clarke attempted to assassinate Lexa at first but grew to trust Lexa again after Lexa vowed never to betray Clarke and to always protect her. When they enter a relationship Clarke changes Lexa making her more merciful and peace seeking which Titus Lexa’s flame keeper perceives as weak and in an attempt to kill Clarke and frame the enemy he kills Lexa accidentally to his remorse and Clarke and Titus are forced to upload Lexa’s consciousness to the Flame.

As with all of the Commanders before her, Lexa's consciousness continued to live on in the Flame upon her physical death. When Clarke took the Flame to destroy A.L.I.E. in the City of Light, her mind summoned Lexa's consciousness to aid her. Lexa protected Clarke from A.L.I.E.'s Cult, and Clarke declared her love for Lexa before they parted ways. Lexa's consciousness survived the fight against A.L.I.E. and remained within the Flame

Then one day Clarke renters the Flame to see Lexa, she runs over and lovingly embraces her, kisses her and holds her tight professing I love you, I love you, I love you! I missed you so much!