Gilbert Syndrome: This condition is characterized by a mild increase in unconjugated bilirubin due to a partial deficiency of the enzyme uridine diphosphate glucuronyltransferase (UGT1A1). It is often asymptomatic and may be exacerbated by stress, fasting, or illness, which fits the patient's presentation of chronic mildly elevated bilirubin without anemia or liver dysfunction.

Crigler-Najjar Syndrome Type II: A less severe form of Crigler-Najjar Syndrome, also due to a deficiency of UGT1A1, but with a higher residual enzyme activity than Type I. Patients have mildly elevated unconjugated bilirubin levels but are generally asymptomatic.
Dubin-Johnson Syndrome: Although this condition typically presents with conjugated hyperbilirubinemia, some cases might be considered if there's a mix-up in the lab results or an atypical presentation. It's characterized by an inability to secrete conjugated bilirubin into the bile.
Rotor Syndrome: Similar to Dubin-Johnson Syndrome but without liver discoloration, Rotor Syndrome is a benign condition with elevated levels of conjugated bilirubin. It's rare but could be considered in the differential for chronic elevated bilirubin.

Hemolytic Anemias (e.g., Hereditary Spherocytosis, G6PD Deficiency): Although the patient is not anemic, conditions that cause hemolysis can lead to elevated bilirubin levels. It's crucial to rule out these conditions due to their potential for significant morbidity if left untreated.
Wilson's Disease: An autosomal recessive disorder leading to copper accumulation in the liver and other organs. Early stages might present with mild liver function test abnormalities or isolated hyperbilirubinemia, making it a critical diagnosis not to miss due to its potential for severe liver damage and neurological symptoms if untreated.
Primary Sclerosing Cholangitis (PSC): A chronic liver disease characterized by inflammation and fibrosis of the bile ducts, which can lead to elevated conjugated bilirubin. Although rare in children, it's essential to consider due to its potential for severe complications, including liver failure and increased risk of cholangiocarcinoma.

Lucey-Driscoll Syndrome: A rare condition causing transient neonatal hyperbilirubinemia due to a mutation in the UGT1A1 gene, but it could potentially persist into adolescence in some cases.
Cholestasis due to Bile Duct Stricture or Choledochal Cysts: These conditions can cause obstructive jaundice and elevated conjugated bilirubin. While rare, they are important to consider due to the potential need for surgical intervention.
Thyroid Disorders: Hypothyroidism can cause elevated levels of bilirubin, although this is less common and usually accompanied by other symptoms of thyroid dysfunction.