Mufasa and Sarabi were mates, ruling over the Pride Lands as its king and queen. Mufasa and Sarabi loved each other dearly, and the two eventually had a cub, Simba. Sarabi was devastated over the death of her mate and thought their son had died too. When it was revealed that Scar had murdered Mufasa she fought in the battle of Pride Rock to avenge her mate.
Mufasa cares deeply for his grandson and is always willing to offer him support and advice, with his wise words always helping, Kion come up with a solution to any problem.
Even when Kion uses the Roar in anger, Mufasa is understanding and prompts him to remember the true reason for his anger, showing that no matter what, he will always love his grandson.
However in Season 3, after Scar ordered Ushari to bite him, Kion had a scar on his face. This caused Kion to lose his temper and lose control of his Roar, resulting in Kion becoming too ashamed of his scar to let Mufasa see him. However, in Long Live the Queen, Rani encourages Kion to talk to Mufasa and reassures him that he doesn't need to be afraid. Kion talks to Mufasa after not doing so for a long time and realizes that Mufasa is not angry with him at all, only that he had worried Kion had forgotten about him.
In The Lake of Reflection, Mufasa understands that Kion is a capable enough leader to give up the Roar and informs him of where he should go. In Return to the Pride Lands, at the wedding of Kion and Rani, Mufasa proudly looks down at his grandson and granddaughter-in-law.
Simba had a strong bond with his father Mufasa when he was a cub and looked up to his father in every way. But when Scar murdered Mufasa it broke his heart and soon he ran away in fear, thinking that it was his fault.
But when Simba was an adult, it was his father's spiritual guidance that eventually convinced Simba years later to reclaim the throne from Scar.
In The Wisdom of Kongwe, when Simba and Kion consulted Mufasa after the revelation of Scar’s return, Mufasa reminds his son that he had previously defeated Scar, and will soon find out how to defeat him again.
Years later, Mufasa and Rafiki had made a plan to unite the Outsiders and Simba's pride together by uniting Kovu and Kiara. Although Simba would later exile Kovu, presumably disappointing Mufasa, Simba would then put the hatred of Scar behind him, thus pleasing his father.
Mufasa loved Scar and cared greatly for him. Although Scar doesn't return the affection, due to his jealousy of his brother being king, though not very strong at first. However, one day he made a fatal mistake which would lead to long-lasting results. One day, after returning from a patrol in the Outlands, Mufasa received a report from his brother, who was then the leader of his Lion Guard, that he had defeated a Strange Lion and cobra, who had attempted to take over the Pride Lands by manipulating him. However, Mufasa jokingly nicknamed his younger brother Scar, after the facial wound he received from a black-necked, spitting cobra during the event. This angered Scar and partially fueled by the venom, brought the power to his head, making him believe that he should be King instead of Mufasa. As a result, Scar plotted to overthrow Mufasa using the Roar of the Elders, demanding his Guard to aid him in his plot. When they refused, in anger, Scar used the Roar of the Elders to destroy them, which caused him to lose the Roar completely, forever, becoming shriveled and horrible.
However, his evil brother was able to kill and betray him the second time around and became king afterward, until the spirit of his brother was able to convince his nephew, Simba, to reclaim the throne and succeeded in doing so thanks to him.
After Scar returned to the Outlands as a spirit with the help of Janja, a descendant of the hyena clan who murdered Mufasa alongside Scar, Mufasa has great faith in his son and grandson that they will defeat Scar once and for all, which eventually comes to pass once Kion defeats Scar once and for all.
Despite being murdered by Scar, learning of his true evil nature, and now seeing him as an enemy, Mufasa does not seem to bear any real grudge against his younger brother. Whenever Mufasa spoke to Kion he never once referred to Scar in an angry tone and never once wished vengeance upon him. He has also not let Scar's hatred affect his good judgment, as he does not harbor any ill feelings towards Kion for sporting a facial wound identical to Scar's (which he had received in the final battle against the Outlanders). It is also worth noting that by planning to make Kovu, Scar's adoptive son and heir, and his granddaughter Kiara, Simba's heir, fall in love, Mufasa helped to fulfilled Scar's dying wish (Kovu being king as Kiara's consort). This shows that despite everything Mufasa still held love and respect for Scar.
Mufasa cared a lot about Nala. In The Lion King, Mufasa, and Sarafina, (Nala's mother) had engaged their cubs to marry when they became of age so they could rule the Pride Lands together. When Simba and Nala were cubs, they got chased by the hyenas. Mufasa came in to save them. When he wanted to speak to Simba alone he asks Zazu to bring her home and admonished Simba for endangering not only himself but Nala as well. Nala helped fight Scar after hearing that he was one who killed Mufasa.
Soon after this, Mufasa's plan to betroth Simba and Nala would come to pass as Nala would become Simba's mate and Queen, and thus Mufasa's daughter in law, and mother Mufasa's grandchildren, Kiara and Kion.
Mufasa knows that Kiara will be the future queen of the Pride Lands. In Can't Wait to be Queen, Kion says he's having trouble with Kiara being queen. Mufasa says that he should listen to his sister and help her even if she's wrong.
Mufasa and Kovu are on good terms as Kovu is Mufasa's grandson in law (as well as his unrelated nephew given that Scar adopted Kovu during his reign). During the events of the "Lion King II: Simba's Pride", Mufasa gave Rafiki a sign to help guide and bring Kiara and Kovu together. After welcoming Zira's Pride into Simba's Pride, Kovu married Simba's daughter and heir, making them both Simba's successors to Pride Rock. Mufasa could never have been more proud of his son. He even smiled at Kovu and the rest of Simba's Pride in the Return to the Pride Lands.
Zazu was a very good friend of Simba's father, Mufasa. When Simba was a cub, Zazu would keep an eye on him for Mufasa. Zazu would also give Mufasa the Morning Report every morning. Zazu helped his pride fight against the hyenas after hearing that Scar killed his good friend.
Rafiki was a very good friend of Simba's father. When Simba was born, he held him up for the Pridelanders to see. Mufasa and Rafiki go back a long way. Rafiki helped fight the hyenas after hearing that Scar killed his good friend.
Years later, Mufasa spoke to Rafiki and advised him to bring Kovu, his brother's heir, and Kiara, his granddaughter together to unite the Pride Landers and the Outsiders.
Although the strange lion and Mufasa have never interacted, due to his death by the hands (or rather claws) of Scar, it's likely that the lion has heard of Mufasa and plotted to overthrow him using his younger brother. However, Scar betrayed the lion in the end, ultimately saving Mufasa's life. Despite this, the strange lion's plans to dethrone Mufasa as King would soon come to pass, as Mufasa would mock his brother for his facial wound, prompting Scar to overthrow him and become King himself, but not for his own satisfaction as Scar would rule Pride Rock with full power, and an iron fist, not sharing any power with lion as he would be long dead, and later Mufasa's son Simba would return and defeat Scar, ending his reign, and reclaiming his birthright, only for Scar to return a second time and take back the throne, before being defeated by Mufasa's grandson, Simba's son, and Scar's successor as the leader of the Lion Guard, Kion.