A custodian is a person entrusted with the custody or care of something.

• Whenever Fundamental Rights are denied or restricted, it can be challenged in the Supreme Court under Article 32;
• The dispute between Centre and State can be settled in the Supreme Court under Article 131;
• the Law declared by the Supreme Court shall be binding on all the courts in India under Article 141;
• For the enforcement of decrees - Article 142;
• President of India consulting Supreme Court under Article 143;
• After all, under Article 13, all laws that are inconsistent with or in derogation of any of the fundamental rights shall be void ( Doctrine of Judicial Review).
• Supreme Court judgments are the law of the land;

Based on the above facts, the Supreme Court takes care of the Constitution. So, simply the Supreme Court is the Custodian of the Constitution.