Mercury is not called the planet of communication for nothing. It governs our ability to communicate and think logically. It's placement in the 9th House for Leo Lagna has significant implications on how they communicate with others, perceive information, and form opinions.
When Mercury is placed in the 9th House, individuals become more intellectual and wise. They gravitate towards exploring spirituality, religion, philosophy and higher education. With Mercury being Lord of houses 2 and 11; houses that represent wealth, money earned from profession or business partnerships and gains from social networks respectively; these individuals are likely to prosper financially through their networking abilities or intellect.
They may have a natural affinity for learning new things quickly, figuring out complex problems with ease and finding innovative solutions to any situation at hand. However due to their restless nature they may not stick to one subject/area of interest for long periods of time leading them to be jack-of-all-trades but master-of none.
As mentioned earlier that Mercury governs how we think giving right balance between rationality & emotions in communication but when placed here in neutral sign state the rationality side takes dominance; resulting in a need to guard against becoming preaching or overly-convinced about one-way thinking or beliefs as it could lead them conflict with their peers/family members who don't share similar interests or thought processes.
With Mercury being natural karaka of houses [4 (home),10(profession/career)]; this placement could set possibilities of attaining success through ones intellect by engaging with an occupation related to Education/training, Counselling/Psychotherapy that requires mastery & use of intelligence as main attribute.
As it aspects the third house- which deals with communications at a local level-among family members/neighbors among others - there would be high chances of good relations between siblings/friends/neighbours enabling an individual in reaping benefits from both immediate family network as well as their wider professional network. Their communication skills could lead to the possibility of a successful career in media, broadcasting, sales or marketing.
In conclusion, Mercury's placement in the 9th House for Leo Lagna could produce individuals who are intellectually inclined, possess a natural curiosity for higher learning and fondness with spirituality/religion. It would provide opportunities of financial gain due to their networking skills or profession gained through intellect. However, while guarding against becoming overly invested in ones thoughts or beliefs they should be cautious of developing arrogance which could damage healthy interpersonal relationships which are equally important in life.